Todd Zolecki of MLB.com reminds us of a rarely used rule that could possibly come into play if Roy Halladay is moved before this year's trade deadline. If a player signed a multi-year extension before October 2006 and is traded during the life of the contract, the player can then demand a trade during the offseason. If the team falls to trade the player by March 15th, he becomes a free agent. Halladay signed a three year, $40MM extension in March of '06.
Javier Vazquez is the only player to take advantage of this rule in recent years, when he demanded a trade out of Arizona following the 2005 season. Since he already has a no trade clause it's unlikely Halladay will end up with a team he doesn't want to play for, but he could use this rule as leverage to get a contract extension after the season. That last part is just speculation on my part, though.