I think at some point hook_validate() wasn't working like I thought it should, so I added in a validate handler to the form. But because I wanted to see if it was a Drupal problem or an Übercart problem, I changed it back. Now it works fine.

If you've got some form_alter that's overwriting the validate handlers, that will prevent the product validation from working. This would be fixed with the latest revision, 845.