How does one make multisignature transactions with the bitcoin client?
Bitcoin version 0.6.1 doesn't have useful multisignature support yet, either in the GUI or via the RPC interface, because there is no support for partially signing a multisignature transaction.

What is supported:  creating a multisignature transaction (using addmultisigaddress), and sending to a multisignature address (either your own or somebody else's).

Testing spending of multisignature transactions-- if all of the private keys required to spend a multisignature transaction are in the wallet, then those coins are added to the wallet's total bitcoin balance and that transaction will be spent if the normal coin-selection code decides to use them for a spending transaction.

Pull request 1456 adds low-level support in the RPC interface for multi-device/multi-person multisignature transactions.