All PS players on non-playoff teams had their contracts expire as of the last regular season game. Most have already been re-signed to future contracts with their old teams, but the rest are street FAs and can be signed to playoff teams' 53-man rosters, practice squads, or to reserve/future contracts (2010 80-man roster protected) at any time.
They are street free agents the day the Patriots are eliminated. I say street free agents, because they're not like normal UFAs who don't have their contracts expire until the beginning of free agency. Practice squad players, as well as any players who are released by their teams in February can sign with anyone immediately. They do not need to wait until March 5th and do not follow the same rules as UFAs (June 1st tender, final eight, etc.)
For example, if the Pats lose this weekend, Nick Moore could sign with the Ravens the next day, either to their 53-man roster or practice squad. He could be signed by the Ravens to a future contract for the 2010 season. He could be signed by the Raiders to a future contract. He could be signed by the Pats to a future contract.