It could also finally be going to a six-man bullpen and getting an actual outfielder on the team.
[WED 21:56 UPDATE]: From twitter, it appears that Stephen Pryor's agent has let slip that Pryor is the one getting the call up between now and Friday. Pryor isn't on the 40-man roster so a spot will have to be cleared, but George Sherrill is still on that roster and can be transferred to the 60-day disabled list.
[FRI 11:35 UPDATE]: Stephen Pryor is the call, and Sherrill is placed on the 60-man. I'm so smart.