The new EU roaming tariff only applies when using a sim from a operator located in the EU. The aim is to harmonize roaming costs within the EU. So it doesn't apply to an AT&T sim.
Secondly, the tariff only applies to voice calls and NOT to SMS, MMS or GPRS/UMTS.
Thirdly, I have noticed that people seem to get charged for absurd amounts of data. It is quite impossible to verify that those amounts have actually been used. I have a theory that the calculation method used by the operators is responsible for charging people for amounts that are bigger then the actual use. It would be interesting to measure the actual use (possibly through a tcpdump) and compare this with the bill. I haven't seen any operator that explains in detail which calculation method (using increments) is used.