There are no legitimate position player prospects at AA or AAA. The only promising starting pitchers at these levels are Jay Jackson (8.19 K/9, 3.72 BB/9, 0.60 HR/9 in 75 IP at AA) and Andrew Cashner (7.44 K/9, 3.23 BB/9, 0.16 HR/9 in 55.2 IP between high-A/AA).
Starlin Castro is not exactly mashing, but is more than holding his own at high-A (.306/.337/.405) while being young for the level. Any word on his defense at SS?
Tony Campana (low-A/high-A), while not a high-profile prospect, is fast (44SB/9CS).
Hak-Ju Lee will be 18 through the end of the season and is off to a good start at low-A (.327/.396/.429, 11SB/3CS, 98 AB)
2009 first-round signee Brett Jackson is off to a solid start at low-A, as well (.289/.448/.400, 45AB).