As Gigom (via the Laptop blog, in my case) helpfully points out, these limits let you watch three hours of hi-def video from Netflix (or whoever) a day...if you don't plan on doing anything else online. Ever. Think about how much TV you'd bother to watch if cable imposed those limits on itself. Somebody in the near future is going to make a mint by setting up a "Netflix friendly" ISP. Maybe Netflix itself is going to have to do it.
The collateral damage for this move is pretty much every broadband-based innovation that's popped up in the past five (maybe ten) years. Backing up your hard drive online, or any of those other nifty cloud-based computing gadgets? Sorry, gotta roll that back. High quality Skype video chat with loved ones halfway around the world? You'll have to plan that out like you were going to have to walk every mile yourself. And that if that 3GB Windows 7 service pack didn't terrify you before, now you've really got something to cry about.
Time Warner users very narrowly dodged a pretty nasty looking bandwidth cap in 2009. They claimed it was because the consumers didn't understand, while I claim we understood it a bit too well. The long and the short of it is that if they thought they could get away with it, they'd try it again, especially with online on-demand services siphoning off their core business. The guts of the whole argument against this is that along with a number of other big-gorilla ISPs, AT&T is charging you through the nose for a service they don't exactly trust you to use, even if you're using perfectly legal services that they just don't happen to like. It's time to make them aware that yes, you have been paying attention, and it's time to stop fighting the future.