Hi John -
Ah, no problem. Kerning is moving glyphs closer or further apart to be more optically pleasing. Imagine each glyph exists in a rectangular box, well kerning nudges the letterforms closer together when that box makes the characters further apart than they ought to be. The prime example is a T followed by an o. The o can nudge over to fit under the T crossbar.
Anyways, I'm on the iPad otherwise I'd drop in screen grabs of MC 18 vs 19. 19 has looser, chunkier letterspacing. I'm set as the default UI font, Lucida Grande, so it's not a font issue. I'll try and put up some images for you later tonight.
Off the top of my head, this would be caused by OpenType tables not being active (unless it's flat kerning in there, which I hope not, cuz, ugh, lame, Apple). If, say, Arabic type also doesn't work, then that might be related.
Again, I'll take a look later and out in some screenshots.
Micah