Or are these the kinds of things that are being asked to test thoroughly? Is there a test plan of some sort, or should we just be doing whatever we can think of on testnet to see what happens?
You are the Quality Assurance department. A test plan is an excellent idea, could you write one up and post it on the wiki and ask for volunteers to help test?
Gavin, will Btcoin-QT support plain M-Of-N transactions and P2SH transactions or will it be etiher-or?
I don't know, the GUI for multisignature transactions hasn't been designed yet. But I imagine it will be simpler to always produce P2SH transactions, just as the client always produces OP_HASH160 transactions and has no option to produce plain OP_CHECKSIG transactions.
I assume that both forms will be supported for multisig payments into your wallet (but detailed discussion on how to support multisig in the GUI should happen somewhere other than this thread).