Problem with columns dropping during resize

I’m having a problem with a new theme where when I resize the window, the right column drops to the next line. You can see what I’m talking about at http://dev.withoutatraceroute.com/ (use FF, I haven’t gotten around to hacking it for IE yet).

Ideally, I’d like the two sidebars to be fixed-width and the middle-column to resize down the minimum width. Failing that, I’d settle for the whole thing being fixed-width,