I'm in France with SFR where the iPhone plans give you 1GB of 3G data, after which the connection is capped to GPRS speeds. I use my iPhone constantly (including listening to online radio for at least a few hours/week), and the most I've used so far in a single month is 934MB. I suspect that this move will mostly affect those who are using unofficial tethering and/or those who stream a lot of video from home.

I still think it's a pretty bad move on AT&T's fault, and is probably an indication that they have secured exclusivity in the US for at least another year.