According to Scott Lauber of the Herald, as quickly as he was called up, Fabio Castro has been sent down to Pawtucket, and Alan Embree has been brought up in his place. Haven't heard any word yet on whos spot he will be taking on the 40 man roster. It could well end up being Castro's, however, judging by both his performance so far this year and major league clubs' apparent propensity to bounce him around.
Of course, as well all know, another 25-man roster move is going to have to happen Sunday, when Daisuke Matsuzaka comes back up, so it will be interesting to see who stays and who goes this weekend.
EDIT: Apparently Kevin Frandsen has been DFA'd to make room for Embree.