The halcyon days of AT&T’s unlimited data plan for the iPhone and the iPad are long gone. Instead, new customers purchasing an iPhone 4 or an iPad must choose between a 200 MB a month plan and a 2 GB a month plan — a change we’re not too happy about. At least existing iPhone and iPad users were given the chance to keep their old data plans if they liked them.

The problem is: iPad users seem to have been sold on a bait-and-switch.

Background: Data plans for the iPad are slightly different than those for the iPhone; instead of signing a 2-year contract, you can pay monthly. Before the new pricing scheme, iPad users could skip a month of the data plan and then rejoin the plan the next month, depending on their usage. It was a fairly good deal.

But it turns out that while existing iPad users can keep their unlimited data plans, if they choose to use a cheaper plan for one month, there’s no going back. They lose unlimited data forever.

When he introduced the iPad, Steve Jobs referred to the original data pricing model as a “breakthrough deal with AT&T.” Taking this deal away from customers is nothing short of a betrayal, from both Apple and AT&T.

Fittingly, a group of iPhone and iPad owners filed a lawsuit against AT&T, arguing that AT&T “baited” them into getting an iPad, only to renege on the unlimited data plan as soon as they were happily surfing away on their new tablet.

The jury’s out about whether AT&T is guilty of baiting customers. But we can say that this is yet another example of some seriously lame behavior on the part of Big Telecom.