Ken Rosenthal has the scoop, and says that two minor leaguers would head back to Minnesota. I've heard RHP Kevin Mulvey and 1B Mike Carp, both from Double-A Binghamton.
The Mets were close to signing Castillo as a free agent a few years ago, but he ended up staying in Florida before being dealt to Minnesota during the Marlins' last fire sale. Castillo doesn't strike me as a particularly useful player anymore, as he doesn't really steal bases, he has never hit for power, his defense isn't great anymore, and he has apparently stopped drawing walks. It ultimately depends on who the Mets will give up, but I don't see a lot of upside with Castillo here.
UPDATE [3:51pm]: Castillo is a free agent after this season, and was a Type B free agent last season. The Stats, Inc. ratings are based on two years worth of performance, and second basemen are grouped with shortstops and third basemen for the purposes of the rankings. It's conceivable but unlikely that Castillo will climb the rankings enough to be considered a Type A free agent. So, Castillo would at least be worth a sandwich pick in between the first and second rounds of next year's amateur draft, should he leave as a free agent.
UPDATE [4:53pm]: Dave Cameron at USS Mariner likes the deal for the Mets:
Remember the Ray Durham heist the A’s pulled off a few years ago, where they got two months of a solid player for nothing of consequence, then got two draft picks when Durham left as a free agent. This is the 2007 version of that. Terry Ryan simultaneously made his team worse now and in the future and got fleeced by Omar Minaya in the process.UPDATE [5:39pm]: Omar Minaya is mum, at least according to Mets.com.
UPDATE [7:02pm]: Rotoworld likes it, too:
We understand why some Mets fans wanted to stick with Ruben Gotay, but Castillo remains the better bet on offense and is clearly better defensively. Considering the Mets gave up next to nothing to get him and likely will reap a supplemental first-round pick if they let him go at the end of the year, this is a big win for Omar Minaya. Castillo will start at second base most of the time and should take over as the Mets' No. 2 hitter. Gotay can play day games after night games. Carlos Beltran will likely go on the DL to open up a roster spot tomorrow.UPDATE [7:09pm]: It's official.