For so long, I've been testing this site in IE5.1 for OS X not thinking that I should check it in IE5.0 / Classic. I assumed there would be no difference in the way the two browsers rendered the page. I assumed wrongly; why did nobody tell me this site ballsed up in IE5, a MAC browser! I think I've fixed the problem; it seems that IE5 is more sensitive about how are ordered in the HTML. Anyway, I'm off to bury my head in the sand.