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