The deal would make sense in a number of ways. Both Lowell and Castillo are owed 12 million for the remainder of their contracts. Lowell is owed all 12 next year, while Castillo is owed 6 million for each of the next 2 years. For the Red Sox, this could get their payroll next year down some after the Beltre signing (remember, it was reported a few weeks back that the Red Sox were within $5 million of their budget for the year). For the Mets, they could help free up money for the strong 2011 free agent class.
Castillo hasn't been a good defensive middle infielder since 2007, but before the last 2 seasons (which were just plain bad), he was about average at worst, and good for most of his time with the Marlins. Offensively, Castillo would be an on base machine off the bench. He doesn't offer anything in the way of power, but he can drive a man in from second and will serve as a bridge to better part of the lineup if the Sox were to get stuck with the bottom of the order in a crucial situation. He would also provide a stolen base threat (which are slowly, but surely filling up the Red Sox' lineup), allowing them to play the late-inning small ball that might be important on a team that will be looking to win low-scoring games next year.
Earlier this year, the Red Sox tried to trade Mike Lowell for $3 million in salary relief, and Rangers prospect Max Ramirez. The deal fell through when it was revealed that Lowell's thumb would require surgery, which was performed successfully a few days ago.
UPDATE [11:20 PM EST]: WEEI has a source saying the deal is not close. They believe the Mets might even have to pick up some of Castillo's tab in any trade. I'd call it a coup if the Sox could make them eat any part of Castillo's deal.