I didn't want the auto-renew either so I took the gamble and hit "Cancel Plan" under View Account. It then gave me the option to delete my account immediately or after 60 days of inactivity. I selected the 60 day option. It gave me a confirmation that auto-renew was cancelled, and my data plan is still active 🙂
After it expires, as I understand it, if I want to re-activate data within those 60 days I can do so without having to re-enter my information.
BTW, loving the 3G ability. Glad I signed up for the unlimited plan. Only two days in and I've used over 150 mb 😀