We had the same problem here, with a lot of our users using IE 5.x on OS X.
I found the problem with the templates. The issue is that IE on the Mac is unable to handle <input type="image"... tags properly.
It dies when you try to do more than one or two in a group.
The current work arround (that we're using now) is to change them into normal buttons; definitely not as pretty, but simple. I suppose another option would be to rewrite the page to use <img> and <a> tags to get the same behaviour - and retain the pretty graphics - although it would be trickier as it would require some additional coding.
Has this been fixed in the unrelease 2.1?