Well, I *finally* figured this out. The problem is, I've been clicking on the Aces High logo in the upper left corner to get to the main page. There is a "&BypassCookie=True" parameter being fed to the UBB via this link. I also had bookmarked this site with that parameter in the URL. When I took out that parameter, the lightbulbs and colored folders work fine.
I'm not sure why that's there, but I've never seen another UBB with that feature. For some reason, only IE5 is affected by this. Can I get the webmaster's take on this?