Thanks Shawn. I have an old full-height 5.25" tape drive enclosure and power supply, but I was going to need to drill a set of mounting holes for my 3.5" hard drive.
Ultimately, I decided the other case was just way too big, so ordered this one:
http://www.geeks.com/details.asp?invtid=SLIM-USB20-35
It arrived today and I think it's going to work out fine, although I'm probably going to have to make a small modification to be able to run an IDE cable to the outside. But the price was right.
I'd still like to hear what other people are using for a drive enclosure.