A friend and I put together a P2SH-based multisignature cosigning web service designed to work with bitcoinqt (on both mainnet and testnet). 

The documentation / examples walk through exactly what commands you need to feed to bitcoinqt to pull off 2-of-2 and 2-of-3 multisig using the service as a cosigner.  The service also simplifies spending from multisig addresses since it tracks unspent outputs for multisig addresses (something missing from bitcoinqt currently) and has functions for creating raw transactions just given a "to address" and an amount.

I'd love to hear feedback from anyone interested in multisig on things we could do to make the API more developer-friendly or on policies you think it would be valuable for a cosigner to enforce.  The policies we currently offer are focused around wallet-security use-cases.