[Note: This article has been superseded by "Ins and Outs of the 3G iPad AT&T Service Plans," 30 April 2010, which was written after working with a release version of the 3G iPad.]

Since Apple posted information weeks ago with preliminary details about the data plans for the 3G iPad - arriving on doorsteps on Friday - available from AT&T in the United States, I've been wondering about what either company meant by "month." My early thoughts are at "3G iPad Will Allow Data Upgrades for 250 MB Plan," 23 March 2010.

A data sheet from AT&T in PDF form makes this all clear. A "month" is a 30-day billing cycle that starts whenever you purchase a contract-free plan. The plans are $14.99 for 250 MB of combined upstream and downstream data in that billing cycle, or $29.99 for true unlimited data. Both plans automatically renew each month unless canceled.

Some sites have speculated since the iPad announcement in January 2010 that the "unlimited" plan would have a 5 GB limit, which is true of nearly all 3G laptop-based services from mobile carriers. However, given that AT&T has an unlimited plan for the iPhone, I didn't see how anyone would try to finagle that. Carriers stopped using the term unlimited except in its actual sense after Verizon agreed to change its marketing a few years ago following an investigation by the New York State attorney general.

AT&T explains that, as TidBITS editor Mark Anbinder thought, a new 30-day countdown starts if you purchase another 250 MB allotment if you run out of data on a current 250 MB plan. That limited data plan is actually giving you either the rights to as much as 250 MB or the use of a 3G connection for up to 30 days, whichever comes first. If you don't hit 250 MB in 30 days, you can't roll over the unused data portion.

Those with a 250 MB plan can also switch to an unlimited plan. You can upgrade to unlimited at any time during a current 30-day 250 MB plan cycle. If you don't use up all 250 MB, the unlimited plan goes into effect at the end of the current 30-day cycle.

But if you use up 250 MB of data before 30 days are up, the unlimited plan starts at the moment you cross the 250 MB threshold, and starts a new 30-day cycle at that date and time.

What this plan allows is a restart of a 30-day cycle whenever you run out of data in a 250 MB plan. This is unique among the big four carriers, which otherwise charge overage fees on laptop data plans if you exceed a 200 or 250 MB plan (depending on carrier) of 5 to 10 centers per MB within a 30-day billing cycle.

The closest offering to the iPad plan is from Virgin Mobile Broadband (a Sprint division), which sells a $99.99 USB modem with which you can buy chunks of data from 100 MB to 5 GB that must be used within 10 or 30 days. When you've used up the data in the plan, you just buy another chunk.