I'm stuck in the same spot with my brand new iPad 2 3G-
I noticed that PREPAID isn't an option turning it on. I'll likely use 3G only for trips, so how would I do this?
Note: I had a gen1 wifi iPad until I upgraded to the 3G version this time.
That's EXACTLY the same situation I'm in - 3G is really only for trips; otherwise I have WiFi. I think AT&T only has the Postpaid option because it locks you in, so you have to pay whether you're using it or not. They probably LOVE when people complain about how hard it is to switch to Prepaid, because it discourages anyone from doing it.
The ONLY way to change or cancel your Postpaid plan (also known as "DataConnect rate plan") is to call AT&T or go to an AT&T retail location. 1-800-331-0500
If you tell them exactly what to do, maybe it'll go smoother for you. You might actually be best off not telling the first person that you're going to want to add Prepaid service, and instead just get them to cancel your Postpaid service first. It might save them some confusion.
1) You want to cancel your DataConnect rate, also known as Postpaid, plan.
2) To find your account, they need your 'iPad phone number' (your iPad actually has a phone number, even though you don't use it for calls) - access it on your iPad at Settings > General > About > Cellular Data Number
3) Once that's done, you need to set up new Prepaid service on your iPad (if you already have an AT&T account, you likely want to set it up on the same account so you can manage your iPad alongside your phone).
4) They will need to send you a new SIM card in order to activate your Prepaid data plan on your iPad. They should be able to waive the shipping fee on the card.
Note that you will not have any 3G service until you get your card. If you need service in-between, you may want to do #3 & 4 first, and wait to cancel your postpaid plan until after you get your prepaid plan set up; but then you have to call them back, and that may be more of a hassle than it's worth.
Here's the only page on AT&T's help section that addresses this issue (and just tells you to call them):
http://www.wireless.att.com/answer-center/main.jsp?t=solutionTab&solutionId=KB117148
Good luck - report back how it goes.