Can you drag from e.g. Mail's compose window w/o problems, or do you  
experience the same deselecting problem there?

No - After a little comparison, I'd say Mail is more tolerant about the ever so slight moves of the cursor coming from the use of the Intuos tablet.  If I'm careful not to move at all in TextMate, it works fine, but if I just work as usual, 1 out of 3 times, it will deselect...  In Mail, it just works perfectly...

I pulled out my old mouse from the box to compare and, as I suspected, I don't have the same problem.  I guess that's enough to confirm my suppositions.

I suggested the idea of a pref for the app so that people who like the default behavior won't see any modification as a regression.
Although the problem might not effect a majority of the TextMate users (how many people use tablets for everything on their computer?), it is still a very fundamental part of the feature set.  That's why I believe it deserves a little attention (read: "slider" :)