These use Sean Smith's system, Total Zone.  He's done all players, all levels back to 2005, so we can start to see some trends.  Important, because one year of defensive data is not generally all that useful, and minor league seasons are shorter than major league ones.

Here are notes on a bunch of guys you might care about.  All the numbers are denominated in runs above average, so +10 means 10 runs above average.  Keep in mind this is largely based on making plays, so while there is, in a sense, a penalty for making a bad throw that pulls the first baseman off the bag and doesn't get the runner, there's no additional penalty for corking it into right field and letting everybody take two extra bases.  So error-heavy guys like Gamel should be adjusted downward.

• Iribarren: Great year at West Virginia in 2005 at 2B, when he was +14, otherwise average.
• Gwynn: Very impressive +19 in 2006, +4 in CF last year.
• Escobar: As noted a few days ago, he was +22 last year, which is either a massive outlier, or he improved a lot, since he was -13 in Brevard County in 07. 
• Gamel: Always negative, including -12 for WV in 06 and -23 in 07 for Brevard County.  He was only -2 last year, which we might attribute to Don Money, but of course we can't forget all those throwing errors.
• Cain: Of all the players I've looked at so far, he's the one whose TZ numbers most clearly point to an above-average defender.  In RF, he was +27 (!) in '06 and +12 in '07 . The +12 is still kind of impressive since he played a fair amount of CF that year, limiting the amount of time in which he racked up the +12.  In CF, he's consistently at or a couple runs above average in smaller samples.
• McGehee: Except for a flukish +10 in 2005, he's the very picture of average.
• Brad Nelson: Just as average as McGehee, only at more positions.
• Ryan Braun: to compare to Gamel, in 06 Braun was a total of -13 split between two levels.  We don't have more than a half-season at any one level for Braun though.
• Bourgeouis: Because he's played so many different positions, we don't have a big sample for any of them.  But from what we've got, looks like a bit above average at corner OF, little below at CF.  In 2005, he spent more than a half-season at 2B and came in at +2.
• Chris Duffy: Consistently a few runs above average in CF.
• Taylor Green: In 06 in Helena, he was +6 at 2B.  In 07 at WV, he was +5 at 3B.  In 08 at 3B he was averae.
• Cole Gillespie: Consistently a few runs above average in the corners.  Keep in mind this is a 'range' stat so there's no consideration of the player's arm.
• Caleb Gindl: It's only one season in low A, but Gindl was +13 in RF and +3 in CF last year.  Worth watching.
• Brent Brewer: As with Gamel, keep in mind that he's worse than his 'range' number.  That said, he's been consistently below average, including -9 in '07.