What I found odd is that the body background colour isn't even defined in the CSS portion. So are you getting a white background outside of the information boxes?
As is, only two sections are defined to have the colour; #5A5A5A, which is not a standard web safe colour, which may cause old browsers to choke, but Internet Explorer 5.1 for Mac had excellent support for CSS 1 and basic support for CSS 2. I'm not sure why you are getting any problems with it however. The source looks all right (though, it isn't officially valid either).
Skyhook - FWIW, Internet Explorer for OS X also has the same problem.
It seems that IE can't handle the CSS and background setting very well. I know that with Firefox (and the excellent Web Developer Extension), I can change the background colour, and it yields the exact same symptom as IE (Except I choose the colour, and why IE chooses blue makes no sense. There is nothing defined as blue anywhere in the source code).