I definitely want it clean and simple, too, though that doesn't
necessarily imply the lack of a slogan/suchlike.
I made clean-and-simple (in my opinion) mockups of just the logo and
"textmate", and the same with "text⇥mate". I actually think I'd prefer
the latter after seeing it "in action". Just one character away from
full simplicity, but adds a lot of spice.
The PSD (Adobe Photoshop) is at
http://henrik.nyh.se/dump/TextMate%20shirt.psd (0.5 MB) if someone
else wants to play. Used Calibri for the font (very close but not
quite what's used on http://macromates.com/, it seems). The t-shirt
graphic itself is from some "make your own t-shirt" site.