Google does reset PR when a domain is deleted.
However, because the Toolbar PR ranks shown for deletes and registrations are only updated every six weeks some trailing domains may still show as having PR when they delete.
The general rule of thumb is to search Google for "info:whatevername.com" and see whether it returns anything. If it doesn't return a result, then the PR has been reset.
I should also add that you should be extremely wary of any PRs you do see listed. For most deleting domains, the PRs they had during their lifetime were forged.