i checked your CSS in the CSS validator, and got a parse error. note at the end of the last class, there is a additional parenthesis and semi-colon. this is the error i got:
Parse error - Unrecognized : );
notice it right above your closing of the style tag. after the last curly bracket: (delete the ); and re-test)
tell us how it comes along... fun site!
FYI: ------- i wrote this below before i saw your site link! i leave it here for reference for others.
it's a shame that IE5.0 is still circulated on Mac OS X. it's woefully out of date. it is most likely the CSS indeed.
look for a backward compatible wordpress theme that uses tables for layout. if you can find one that suits.
or, are you handy with CSS? replace the layout dividers of your theme with tables. here's some how-to's an a little more on the topic of tables for layout. sometimes it must be done!
even your man mezzoblue from the CSS zen garden recommends tables in some cases, like yours. you don't have to go nutz and nest tables, and use spacer gifs! just a simple framework to control display.