AT&T iPad 3G details
We will be taking a couple short trips this year, and now that I have a cellular capable iPad it's time to figure out how I'll have Internet access. TidBITS has covered the 'new' AT&T Personal Hotspot feature on iPhones, and the trade-offs between iPad 3G service vs. connecting through an iPhone hotspot). I don't travel much, so I prefer AT&T's proratable service over Verizon's monthly service. I got the AT&T 3G iPad, rather than the Verizon 3G model or the WiFi-only model.
I thought I would link my iPad to our AT&T family plan, so I could start and stop service with myAT&T.app, but normal iPhone service (including our family plan) is under AT&T's 'postpaid' model, where each month they bill us for minutes/texts/features actually used. The prorated 3G service is prepaid, and doesn't appear to link up with iPhone service or be manageable through myAT&T.app.
Our first trip will be a long weekend with family. In Settings.app, under Cellular Data, I enter my billing info and sign up for the 250mb/30-day plan for $14.99. Hopefully that isn't prorated out to 8.33mbytes/day! The second trip will be over 8 days so I'll sign up for the $25 2gb/30-day plan. Signing up automatically activates autorenewal, so each time I will stop autorenewal after I sign up, and terminate service manually when we get back home. Apparently you can register for AT&T 3G service even without an active contract or WiFi service -- even without a contract the iPad has access to an AT&T (or Verizon) captive network where you can just sign up -- but I don't intend to test this.
Supposedly http://att.com/ipadlanding provides a web interface to manage iPad 3G plans, but it won't accept my (postpaid iPhone) credentials, and I don't see any reason to register again.
Had I chosen to use Personal Hotspot on the iPhone instead of iPad 3G service, I could have changed from my current "DataPro 2 GB for iPhone" service to "DataPro 4GB for iPhone" (which enables Personal Hotspot) directly in myAT&T.app. Alternatively, an AT&T rep offered to schedule my changes to 4gb/tethering and back to 2gb (no tethering) on whatever dates I prefer, over the phone.