After much fruitful discussion and evolution, this proposal is now a series of "Bitcoin Improvement Proposals" -- see BIP 11, 12 and 13: https://en.bitcoin.it/wiki/Bitcoin_Improvement_Proposals
Sincere thanks to everybody who contributed improvements, ideas and code.
Btw, @gavinandresen, I forgot to say thanks for all your hard work :)
EDIT: for some reason the reply I sent by my phone (prior to your last comment) didn't post to github. Here it is:
Like most things, this could have been avoided with better communication. The following would have done nicely:
"Support for this minimum feature set is needed for 0.5's improved security features. The security of the whole network is improved by adding these transactions immediately. Support for generalized m-of-n transactions will be added as soon as a consensus is reached, but we deemed it necessary to split the process into two stages so as to get these new security features out the door ASAP."
FYI, for the future.