Apple and AT&T are finally releasing more details about the iPad 3G's data plan options (PDF). We already knew there'd be two options, neither of which requires a contract: $14.99 a month for 250 MB of data, and $29.99 a month for unlimited data. But now we have more specifics about the actual plans and how they'll work.
If you decide to go with the $14.99/250 MB plan instead of the unlimited plan, your iPad will let you know when you're about to reach your limit. According to AT&T's data sheet, you'll get three alerts: at 20 percent, 10 percent, and 0 percent. With each alert, you'll be given the option to add more data (or you can just wait until you run out).
If you do run out of data, you can either add another 250 MB (for another 30 days--your "month" will just begin again), you can upgrade to an unlimited plan (again, your "month" will be 30 days from the time you upgrade), or you can wait until your billing cycle renews (while there's no contract, your cycle will renew every 30 days unless you cancel it).
iPad users with a cellular plan will receive unlimited access to any of AT&T's Wi-Fi hotspots for as long as you have an AT&T data plan--a nice plus for both customers and AT&T, as customers using Wi-Fi will put less strain on AT&T's network. (In case you're wondering, iPhone customers with an AT&T data plan also currently receive this complimentary access to AT&T Wi-Fi hotspots.)
It appears that AT&T and Apple are doing everything they can to make the iPad 3G data plan painless for customers. You can cancel, upgrade, or downgrade your service at any time from your iPad by going into Settings>Cellular Data, with no penalty.
Now, if only Apple would make a 3G iPod Touch and offer the same contract-less data plans from AT&T...
The iPad 3G will be available in Apple retail stores beginning at 5 p.m. Friday, April 30. Customers who pre-ordered 3G-enabled iPads will also receive their tablets on April 30 (though, customers who pre-ordered 3G iPads after April 19 will not receive their tablets until May 7).