You can select a linear selection as normal (with the keyboard) and then just tap the option key. It will turn the linear selection into a columnar selection by using the start and end points of the linear selection as opposing corners of the block. Tap again to go back.
That is probably my favorite-yet-obscure TextMate feature.
And it's truly painful when I realize (too late, after muscle memory did its thing) that it's not a universal feature that all text editors have.
+dru
This is a very nice tip, thanks!
-Jenny