I personally have no idea how to execute a multi-signature transaction today. Maybe all I need is a tutorial.
The backbone is all there, but there's no friendly GUI, and you have to use the "raw" transactions, which require understanding quite a bit more about the internals of Bitcoin than you usually need to understand to just make a payment. In particular, you need to figure out which transactions you want to spend, and make sure that you spend as outputs all that you intend to spend since anything you don't spend is taken as a transaction fee. You need to either use the command line (using bitcoind and have the JSON-RPC server set up in your bitcoin.conf) or use the "Debug console" in the Help menu of Bitcoin-Qt.Information on raw transactions: https://en.bitcoin.it/wiki/Raw_Transactions
Here's an example of a 2-of-2 signature setup on testnet: https://people.xiph.org/~greg/escrowexample.txt
Some notes from Gavin: https://gist.github.com/3161162
An example of a 2-of-3 transaction: https://gist.github.com/3966071
So there's some stuff scattered about, but I don't think an easy "here's how to do multisig tutorial" that I've seen. I've played around with it quite a bit on testnet, so I think I have a pretty good handle on it. Maybe if I get a chance I'll write something up. But I probably won't get a chance for quite a while.
