The big problem with predicting call-ups is that the still murky labor situation might make some clubs reluctant to bring up their youngsters. But let's assume that an agreement will be reached; in that case, the following potential impact players come to mind:
Carl Crawford should be called up at some point this season. He has lots of talent, but might take a while to adjust to the majors.
Brett Myers is probably the best pitching prospect in the minors right now. The Phillies have no immediate plans to bring him up (ditto for Marlon Byrd), but I'd be surprised if both of these guys don't at least get to sniff the big league air later on.
It's probable that Jack Cust will be brought up at some point, either by the Rockies or, less likely, following a trade to the AL.
If Jason Kendall is in fact traded to Colorado, grab J.R. House immediately, even if Pittsburgh receives a catcher as part of a deal.
Check out our Point of View article as well for a list of top prospects. Some of these might not reach the bigs for some time, but they're all worth keeping an eye on.