See this comment from Gavin : https://bitcointalk.org/index.php?topic=82213.msg906833#msg906833

Edit : Basically (if I get it right), you can make a multisign Bitcoin address using the command line (but no user-friendly GUI). And to spend funds from this address, many seperate people have to sign the transaction. So this is not as simple as sending a transaction from your computer. Some work still need to be done on this part.