I don't know, but here's a little speculation...
I wonder if it's got anything to do with you not having a value attribute written in that input element. It's probably not that, because if it were,...
With the IE Developer Toolbar you can see what the browser sees. If you put a closing 'ul' tag there, the browser sees an opening 'ul' tag. If you put a fictitious closing 'all' tag there, that's...
I think the point is that you're wasting your time testing on anything less than IE5 because nobody uses it/them; even with IE5, it's pretty much a waste of time testing there too.