The Satoshi client has support for multi-signature transactions through the JSON-RPC API since version 0.6.1rc2.
My understanding is that a multi-signature transaction creates an address which is special only in that spending from it requires multiple signatures, and its prefix is '3' instead of '1'.
As Gavin said on bitcointalk.org, "you can then send funds into that 2-of-3 transaction using the normal sendtoaddress/sendmany RPC commands, or the GUI (or anything that's been updated to recognize multisig addresses)."
What I read from that is that all wallet clients and web wallets might not yet support sending to addresses with a '3' prefix. How wide is that support currently?