Apple started taking pre-orders for the iPad this morning, and provided more details of the optional 3G data plan: Apple's deal with AT&T, assumed to be somewhat similar with non-US carriers when those plans are announced, called for $15 price tag on 250 MB of combined upstream and downstream data usage, and $30 per month for unlimited usage. Neither plan required any contract commitment.
[Update: To expand on a question and my answer in the comments, Apple and AT&T market the iPhone and iPad 3G unlimited plans as truly unlimited. Average iPhone user is a few hundred MBs per month; heaviest use is likely on Wi-Fi networks. With no tethering, an unlimited plan isn't a huge risk for a carrier. If AT&T ever lives up to its promise to offer tethering, expect a separate fee and 5 GB limit.]
Apple made clear today that the 250 MB plan doesn't tack on overage fees when you exceed that quantity of data: rather, you can either upgrade to unlimited (presumably for $15, but that's not stated), or shut off 3G when you run out. That's the most humane offer I've seen to date.
As stated at the iPad launch announcement, iPad owners with 3G built in ($130 more than the Wi-Fi-only version) can sign up for a plan or turn it off from the iPad without having to visit a Web site or go through a separate process. Reducing friction always improves sales, and that also dramatically reduces AT&T's costs by making it a self-service, Apple-handled option.
All four major US carriers offer a low-bandwidth option for 3G service in which 200 to 300 MB of usage is included, but extra megabytes are charged at 10 to 20 cents a piece--$100 to $200 per GB. Virgin Mobile is the only firm that lets you buy preset chunks of data (which must be used within either 10 days for the smallest increment or 30 days for the three larger options).
Apple says that 250 MB subscribers will be warned when they have 20, 10, and no data left, and can then choose to upgrade. Because the plans are month-to-month, a subscriber could upgrade one month and return to the 250 MB level the next.
The lack of "gotchas" will definitely go a long way in getting more people to buy the 3G model and use the service.