FWIW I was just playing around with p2sh addresses on a whole lot of things. For those who haven't seen one, they look like this: 3FArYwLzPwRBbh1yN3Fx6tBZ5rZBPdaMPt the difference being the 3 at the start. A lot of software out there seems to choke; I messed around for a bit asking various websites to make payouts and I'd guess support was about 50:50. Blockchain.info and blockexplorer both can't recognize them, showing up as "Unable to decode output address" and trying to search for a transaction with such an address in it fails, and using such an address as an input to a transaction to, say, bitlotto or satoshidice, is apparently rather ill-advised...
BTC Guild appears to have no problem validating and sending transactions to p2sh addresses. I just sent 0.01 to 3FArYwLzPwRBbh1yN3Fx6tBZ5rZBPdaMPt as a test and the transaction is in queue waiting for confirmations.
UPDATE: Transaction has been confirmed and is part of the blockchain.