All these guys could just as easily arrive as middle relief (probably better likelihood of than not), but here's ranking from top to bottom:
Jose Capellan (Braves)
Carlos Hernandez (Astros)
Dennis Tankersley/Tim Stauffer (Padres)
Jesse Foppert (Giants).....will want more next year than this
Jeff Francis (Rockies)...awesome in minors this year, but Coors effect brings him down -- most likely to start of this bunch, but wouldn't say he's the most likely to get a Sept. callup.
JD Durbin (Twins)
Felix Diaz (White Sox) ... starting this week
Jon Rauch (Expos)...now up
John Maine (O's)
Rick Ankiel (Cards)...I just wanted to put his name here. He'll be used as a reliever in Sept.
Jason Arnold (Jays)
Denny Bautista (Royals)
Scott Baker (Twins)
I don't think he makes more than one or two relief appearance (at most), but Tampa's Scott Kazmir is as talented as any of the above.
He's strictly a reliever, but should have K/IP+ upon arrival to the bigs:
Jairo Garcia (A's)