Tears. Apparently "principles" are not worth $350 then. The bottom line is that unless you put your money where your mouth is you've got no leg to stand on. Those that don't clearly don't have that much of a problem with the new terms and conditions.
I'm not a lawyer, but if you can't wait for the inevitable class-action lawsuit (and I can't imagine any consumer attorney passing this one up; it's probably worth $1M or more in attorneys fees, and is a dead-bang winner), then pay the $350, and then sue AT&T in Small Claims court, where you'll almost certainly win.
If you live in California, you may be able to sue AT&T for violation of the Unfair Competition law, and that will allow you to sue for a civil penalty of 2X the $350, so your total lawsuit would be for $1,050.
If you've been throttled before, and was told you couldn't cancel unless you paid the ETF, you probably don't even need to pay the ETF to sue; your damages start from the day they wouldn't let you cancel without paying the ETF. If it was six months ago, just take your last six months of payments to AT&T, tack on the 2X civil penalty, and sue for that.
In California, an individual can file a Small Claims lawsuit for up to $10,000. It's no class-action, but it ought to get their attention.
Keep in mind that, while Small Claims courts lean in your favor, if you lose, you won't be able to appeal. The case will be over, and you'll have no other recourse against AT&T.
You COULD use hundreds or thousands of dollars in your pocket, while simultaneously sticking it to AT&T, right?