Nothing custom other than dungog.net's chroot panel.
I removed slimserver, restarted httpd-admin, and reinstalled slimserver... and now it works!
I had a friend who knows the SME server inside and out come over, and we were able to duplicate the problem by running the slimserver rpm from within an ibay that contained a "default" index.html file (when I originally downloaded it, I had it in a fresh ibay I'd just created. The second time around, I moved it to a directory I have simply called 'installers')
Today I'm going to get to the bottom of this... I'm going to reformat the whole thing with 6.0.1, and try again.