[quote name='javeryh']OK, it isn't showing up in Disk Management at all... any other ideas? Is there a way to check what is plugged into a USB port (the port works definitely)?[/QUOTE]
First, I would try the drive on another PC to rule out the PC altogether.
If you put this drive together yourself (bare HD + case), you might want to just try reseating the drive within the case. Just open the case, disconnect the HD from the case electronics and reconnect. If you bought the external drive as a whole, then you can still do the same thing, but warranty issues etc. come into play. While you've got the case open, check to make sure that you can hear the drive spinning up when you turn on the power to the case.
If that doesn't work. It might be a problem with the external enclosure. To test this, you'll need to remove the HD from the case as before, but then mount the drive directly into a PC as if you're going to install it internally. See if you can the the PC to recognize it that way. Alternatively, if you can get your hands on another external case or IDE->USB adapter you can try that HD with that to rule out the case being the problem.