IE6 and 7 have issues with floating divs.

http://www.google.com/search?hl=en&sa=X&oi=spell&resnum=1&ct=result&cd=1&q=ie6+float+bug&spell=1

http://www.google.com/search?hl=en&q=ie7+float+bug

I won't go into IE5 becasue if surfers are still using IE5 than they are beyond help.