I think this is one of the hardest things to predict, which is why it's usually not advisable to draft relievers unless they have a clear opportunity to close games.  That being said, there are a lot of young and interesting high-upside arms that are definitely worth tracking and could find their way into a set-up or closer's role in due time.  Just look at Joakim Soria last year.  So here's my list of potential diamonds in the rough for relief pitching  mostly those who are expected to be on the opening day roster that may someday become truly valuable:

Brandon League - Old hype has died, still has high-90's heat and had put it together in 2006 before derailed by injuries, Jannsen's labrum tear gives him shot at set-up or 7th

Fernando Cabrera - I'm not willing to call him a bust just yet.  Struggled and released in 07, but improved in BAL and has better path to closer's job if he pitches well

Ross Ohlendorf - Don't know much about the guy, but good #s and if Joba goes back to the rotation midseason, may have opportunity in aging + shallow bullpen

Denny Bautista - Struggled bad in 06, but former starting top prospect has triple-digit FB and has been lights out this spring, injuries give him path to at least set-up

Jim Hoey - Has been great in minors, hyped as next closer, hasn't put it together in majors, confidence issues? Only 25, will likely get another chance

JP Howell - Rotation looking more like a long shot initially, has done well in ST but struggled last year, will he do better in pen?  Running out of chances to impress

Santiago Casilla - Former top relief prospect did reasonably well in 07 stint, made case for possible set-up role.  Trades of Street/Embree would give him big chance

Phil Humber - Not sure if he'll be in minors or pitch in relief, former overhyped Mets prospect can still be #3 in rotation, but could be solid in pen, good in ST

Jimmy Gobble - "Cac-Toe" quietly posted a great season for the Royals in 2007, though K rate fell. Set-up role is open in KC, more is Soria starts or is traded

Jensen Lewis - Has good mid-90's stuff, debuted in late 07 and was very solid in season and playoffs.  Blocked from top roles in pen, but younger than the others

Feel free to discuss any other interesting relievers I failed to mention.  Happy Purim!