Is Carpetbagging Worthwhile?
My own view currently is that with interest rates at historic lows and an apparent lack of competition between financial organisations we need more not less competition in the banking sector. Whether the very small building societies can offer this competition i think is debatable as non executive directors fees seem to take out a lot of the profit that could be used to offer attractive rates to both borrowers and savers so i would like to see consolidation from the smaller building societies but purely from the stance that as a part of a larger society they might be able to offer better rates.
In the past it was financial beneficial and for the average carpetbagger who opened an account in the hope that he or she might get a windfall should the building society board decide to merge with another society (without trying to force it through at an AGM) i think it was a shame that they were so often vilified.
Nationwide Building Society is a good example of how competitive building societies can be and their current fixed rate bonds are offering some of the best rates available. 

What is a charitable assignment?
This is how the building societies association describe it

"In order to deter carpetbaggers and protect genuine investors, many societies have introduced a charitable assignment clause to application forms for new savings accounts. This means that, for a specified period of time, or in some cases indefinitely, any possible windfalls the member may receive through conversion to plc status would go to a charity chosen by the building society."

Which Building Societies do not have a charitable assignment clause and allow you to start carpetbagging assignment free?
CATHOLIC BUILDING SOCIETY (merged with chelsea) 

What is the minimum investment i need in a Building Society to get a carpetbagging merger payout?
To obtain membership rights the minimum required is normally £100 in a share account. Most Building Society accounts are share accounts.  

Which Building Societies have merged recently and paid a windfall to carpetbaggers?.
Staffordshire Building Society merged with Portman Building Society minimum payout of £200 to qualifying Staffordshire members
Lambeth Building Society merged with Portman Building Society minimum payout of £400 to qualifying Lambeth members.
Mercantile Building Society merged with Leeds Building Society minimum payout of £100 to qualifying Mercantile members.
Universal Building Society merged with Newcastle Building Society minimum payout of £400 to qualifying Universal members.
Portman Building Society merged with Nationwide Building Society minimum payout of £200 to qualifying Portman members.
Catholic Building Society announce merger with Chelsea Building
Catholic Building Society Payouts.
Savers (account balance)
£100-£2200 payout £100
above £2200  2.2% of balance upto a max of £500
Derbyshire Building Society & Cheshire Building Society Merging with Nationwide Building Society. (no payouts due to financial climate!) 
Barnsley Building Society merging with Yorkshire Building Society. Windfall possible if litigation/ collection of the £10 million successful.
03/11/2008 Scarborough Building Society merging with Skipton. (no member vote or payout)
21/01/2009 Britannia merging with Co-op (members vote)
Chelsea BS announce merger with Yorkshire Building Society
Chesham announce merger with Skipton Building Society

23/03/2010 Coventry Building Society announce merger with Stoud & Swindon) (no payout)
Norwich & Peterborough BS merges with Yorkshire Building Society (no payout) 

How safe is my money if i start carpetbagging?

The Financial Services Authority (FSA) has today increased the compensation limit for bank deposits from £50,000 up to a total of £85,000 for each customer's claim. Customers with joint accounts will be eligible to claim up to £170,000.

Hector Sants, FSA's chief executive officer, said:

"There has been extensive debate about the compensation levels. In the interests of providing clarity over the minimum level for the long term we have now decided to implement the move to a £85,000 limit.

The Government will shortly be introducing legislation to further enhance consumer confidence in the banking sector.

The FSA is also to consult on further reforms, including considering whether the compensation limit should be higher still; the speed with which the FSCS can pay compensation; and the rules surrounding whether deposits are covered on a legal entity, a 'brand' or an 'account' basis.

This will provide effective long-term compensation arrangements in which consumers can have confidence.

The changes to the compensation limit are detailed in the  Financial Services Compensation Sheme Review of limits consulation paper which is published today. The paper also sets out proposals to improve the overall scheme and to ensure consistency in respect of compensation limits for investment, insurance and home finance.

Does the charitable clause mean i will not be eligible for a carpetbagging merger payout or demerger windfall?
No it does not and this link explains.

Loophole aids carpetbaggers' comeback re Charitable clause

Where can i find details of all the mutual building societies in the United Kingdom if i want to start carpetbagging? 

Details of all UK Mutual Building Societies here

Where did the names, Carpetbagging, Carpetbagger and Carpetbag come from?
The term carpetbagger originally derives from the American Civil war and was the name southerners gave to northerners who moved to the south arriving with their carpetbags to profit from the unsettled conditions there.
In the 1990,s the name carpetbagger was given to those who joined (mostly) mutual building societies with the goal of gaining a windfall bonus. The word was first used in this context in early 1997 by the chief executive of the Woolwich Building Society, who announced the society's conversion with rules removing the most recent new savers' entitlement to potential windfalls and stated in a media interview, "I have no qualms about disenfranchising carpetbaggers." Ironically  when Woolwich floated the average windfall for the carpetbaggers was £2175

Why have recent Building Society mergers not resulted in a carpetbagging merger windfall?  
Contrary to the myths some building societies would have us believe, the recent credit crunch has shown that the board of directors of a small minority of building societies have also not been as financially cautious as they would have us believe they were and like some of the banks seem to have got carried away during the credit boom .  
Building Societies are owned by the members so to get around the thorny issue of the members (owners) voting against the proposed mergers they have come up with this little known clause (extract below)
In order to ensure the merger is completed as quickly as possible it will be facilitated under sections 42B(3)(b) of the Building Societies Act in relation to The Derbyshire and 94(5)(b) in relation to Nationwide. As a result the merger will proceed on the basis of Board resolutions of the Derbyshire and Nationwide Boards, rather than by member vote. The Societies’ Boards believe it is in the interests of their members that the merger completes as quickly as possible to avoid a period of prolonged uncertainty.
The finally irony is that the Cheshire board of directors now believe that a merger with no financial windfall for the members was the right thing to do but in 2006 it leaked out that "Cheshire Building Society had turned down a take-over approach that could have resulted in windfall payments of about £650 to members and charities. Portman Building Society, the third largest after Nationwide and Britannia, made a bid last year that potentially valued the Cheshire at up to £300m. Jason Gaunt, the Cheshire marketing director, said  "The offer was unsolicited. We considered the approach and the board's considered view was that it served no useful purpose. It is best for our members to continue as a mutual."
So 3 years later the board of directors would have appeared to cost the Cheshire members a substantial amount of money. So it is not just banks that are good in these current times at destroying shareholders/members/owners value. On a positive note though Nationwide does regularly offer a great range of high interest savings accounts so savers with the Cheshire might well get a better deal. 

guide to carpetbagging UK building societies.