Yankees/Red Sox is the best rivalry in sports, in my opinion, so it's always shocking to see players on both sides of it. Of course, it's not exactly a rare occurrence. From Babe Ruth through Johnny Damon, there's player movement between the two teams just like any other two teams.
Pettitte has started 30 games against the Red Sox in the regular season, and three more in the postseason -- including two games in the 2003 stomach punch ALCS where the alleged curse continued.
And now, the Boston Red Sox are interested in the lefty, just like they were in 2004. Given their current starting rotation, he'd fit as a nice fourth behind Josh Beckett, Daisuke Matsuzaka, and Jon Lester. He'd also provide a veteran presence with significant postseason experience, as opposed to them heading into the season with both Justin Masterson and Clay Buchholz in the rotation.