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.
@maaku: simpler? Compare my patches against groffer's, or write out an (a and b) or c transaction using both proposals. I disagree that groffer's is simpler; maybe simpler conceptually, but much more complicated to implement and (more importantly) test.
As I said, supporting 3 simple transaction types for wallet security and backup does NOT preclude also supporting more general m-of-n cases in the future.