I would not be surprised if this DOES violate some law, and I do expect a big, messy class-action lawsuit.
Think through the math. If some class action firm would even take the case, there isn't enough of a prize involved for it to be "big" or "messy". The only people who could participate in the claim would be 3G buyers between Apr 30 and (let's say) June 7. That's not that big of a group.
To qualify for a piece of the action, they would need to demonstrate some kind of monetary loss. That means only those who could demonstrate that they regularly download more than the new deals 2Gb/month arrangement could potentially play. All those who can't demonstrate that will
save money with the new 2Gb cap pricing plan.
If we believe AT&T when they said 98% of users don't download more than 2Gb via 3G now, then one might suggest that 2% of the iPad 3G buyers might actually have a claim to that action. How many 3G iPads have been sold in the U.S.? Those who have purchases on order now could cancel if they don't like the new terms. Apple has a 14 day MBG, so those who have received their iPad 3Gs in the last 13 days could cancel too.
Given the 2M units sold, let's speculate that that would leave 500K iPad 3G buyers in the U.S. 500K times 2% = 10,000. 10K claimants is not a very "big" group, and the total money involved is not very "exciting" for a class action firm. The "prize" would be estimated based on financial losses resulting from classic use (proven use above 2Gb/month in the past), as no court is going to allow a bunch of people to claim they were going to go nuts with Gb use in the future to justify a bigger "prize". Deductions against the prize would apply the 2Gb/month deal savings, etc.
End result: I can't imagine a scenario where a successful class action suit for this could yield more than $100 for each claimant, and would expect it to more likely yield a few dollars (like past class action settlements) probably paid in Apple gift cards.
For class actions to be big and messy, there has to be either a very large number of claimants and/or a very large amount of "damages." In this case, there would be neither, resulting in very little net prizes for those who participate.
Yes, that's pretty crappy. But welcome to the wonderful world of 3G exclusivity and no contractual obligation for buyer
or seller.