I have seen the same thing lately. I could not determine the cause or reliably reproduce the error, but perhaps it is associated with the latest update (I use the "advanced" build 1631). In my opinion, I started to see the problem after the update (in mid-March).
Sometimes restarting TextMate fixed it. Sometimes I also cleared the font cache.
As Allan noted in the ticket I applied for , this is due to a bug in OS 10.6 with external LCD panels from other manufacturers:
In this case, the problem is probably related to the automatic font smoothing selection (based on the display), see http://hints.macworld.com/article.php?story=20090828224632809 for more details and a possible workaround.
The reason that restarting TextMate sometimes worked for me (and you apparently) is because we restarted it without connecting an external monitor. Running the default command in this Mac OS X Hints article fixes it correctly.
defaults -currentHost write -globalDomain AppleFontSmoothing -int 2
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