Yes. I've read threads where people are doing this. Check HowardForums: Your Mobile Phone Community & Resource
and forums.cingular.com there are threads there. You won't get visual voicemail.
AT&T said, when I called, if they determine a iPhone is being used, they'll add an iPhone plan to the account. You can always argue then they need to credit you for the BB plan but they didn't give me an answer. I still may do it too, but right now the Bold is a pretty nice phone.