According to Allan Odgaard:
Control-T already does that in TextMate.
Uh, where did I get the idea it didn't?
Additionally, if you are at the beginning or end of a line, it will swap the current/next line.
If several characters are selected, it will reverse these. If several lines are selected, it will reverse the order of the lines.
Feature with a nice twist, as usual with TM. Thanks a lot Allan. I've never felt 40 US$ so well spent.