The best argument for legal potentials is this concept of the promise of getting to start, cancel then start again with $30/unlimited when needed. However, if you don't hold a contract from AT&T that grants you that service for that price, they don't have much- if any- obligation to deliver it. As others have said, contracts go both ways. The benefit in not having a contract serves both buyers and sellers. In this case, no contract means AT&T is obligated to nothing going forward, just as buyers are not obligated to giving AT&T money for nothing... or something.
But those of us who already bought an iPad 3G DO have a contract that has a term requiring AT&T to allow us to start-stop-start unlimited. It's not a term of the monthly contract, but it's a term AT&T imposed on themselves. They advertised it. In response to their ad, we ordered iPads (in contract law terms, we made a contract offer, the terms of which corresponded to AT&T's stated terms of service, including start-stop-start. ) AT&T billed our credit cards and activated our service. Hence they accepted our offers, and a contract was formed. A contract need not be in writing, and start-stop-start need not be in the written terms of the monthly activation contract for it to be binding on AT&T. Further, every contract has an implied term of good faith and fair dealing. So while AT&T can argue that it was not obligated to maintain start-stop-start at $30 forever, they cannot argue that they can stop it before we could even do it ONCE.
Those who haven't already activated with AT&T may not have a contract with AT&T (I could make a legal argument that they do), but they have a contract with Apple that requires start-stop-start.