Hi, you're right, I didn't really consider IE5/mac when doing these changes. In fact, I generally don't really give IE5/mac much consideration.
I'm sure someone is gonna disagree with this attitude - accessibility for all is certainly not something I want to underrate - but practically speaking, how many users to you realistically expect to visit the site in question using IE5 on a mac? Microsoft ended support for it in 2005, and if I'm not mistaken it hadn't been updated for years by that time, so it really is pretty ancient. Without having done any research on this, I would expect that many sites these days are somewhat broken or even completely unusable in IE5/mac.
If the site is useable in IE5/mac, albeit visually a bit broken, I would easily settle for that, rather than go out of my way to accommodate the mistakes that some Microsoft programmers made a decade ago and give IE5 users one less reason to upgrade to a modern, fast and secure browser.
Having said that, I'm sure it is possible to fix this to display correctly for IE5/mac too, if you feel it is worth the effort, or you just see it as a challenge You could have a look at using conditional stylesheets to target only IE6 and IE5 respectively perhaps?