I just got my first bill w/ new plan. I got my droid on 11/6 and the DBag salesman screwed up my account.
My billing cycle ended 10/25, from then until 11/6 I was on a dumb phone plan. However, I received a credit for my original plan and was then charged at my new higher rate for 10/25 to 11/25. I should have been prorated for that time period at my old rate. I called and raised the issue and received a $24 credit. The woman on the phone kept saying the salesman "back dated" my account, instead or prorating it. Not surprising considering the guy was an ass, and more focused on helping the customer of the sales girl next to him rather than helping me.
Review your bills!