What escrow system does the official Bitcoin bounty progam use? There is a list of bounties on the wiki here:

https://en.bitcoin.it/wiki/Active_Bounties

How are the BTC pledges collated and eventually paid? I assume there is some kind of escrow system but I can't find out anything about it.
Multisignature transactions can act as escrow.

A transaction could require a party and a trusted third party to complete, or it could be the trusted third party alone who signs to complete the transaction.