I was goofing off in the FireFox web developer plug in (
https://addons.mozilla.org/en-US/firefox/addon/60 :: highly recommended) and came up with this:
I tried to clear out some of the css to see if I could see what the problem is and ended up giving the #maincol, #col2, and #col3 explicit widths and heigts. I then made sure the header was full width so that #maincol would wrap to a clean line.
Anyway, with 3 fixed width columns floating in the page it's a little odd. As the browser window is resized the right columns start to bounce down to their own line in the layout. I'm not sure if this is the effect you were going for.
BTW, this has NOT been tested in IE. I was just curious if I could get FF to work again.