No contract means neither party is under any obligation once the original sale is consummated. You are not required to continue service, ATT is not required to offer service at any set price, or at any price at all.
However, it is unknown what kind of contract ATT may have with Apple. It may be, and probably is the case, that ATT is required to maintain the price levels and service for X year(s) as requirement for being an Apple partner. If ATT jacked up prices in 4 months, it would certainly hurt 3G sales and give Apple a black eye, especially since the iPad isn't compatible w. T-Mobs 3G service. So I'm sure Apple has well protected its brand as it always does.