And I'll be damned if I can figure out what's doing it. I know why those columns move left, and I'm not too worried about it. But for some reason several DOM boxes are overlapping that shouldn't be, and the contents of some of the boxes are set about 15px to the left of their containers.
By the way, Firebug is my favorite new dev tool. I don't know how I ever did CSS without it. And I definitely couldn't have developed against a WordPress framework without it. But even with Firebug, I can't find what's doing the odd left-shift magic.