Depends on the phone. Rogers LTE network is AWS (1700/2100) only. AT&T is either totally or mostly on 700 Mhz now, with plans to move to its AWS (1700/2100) spectrum.
All AT&T LTE phones support 700/AWS. Some Rogers phones, like the One X support 700/AWS, some only AWS. The phones with both 700 and AWS will work on AT&T LTE now. The phones with just AWS LTE will work on AT&T's LTE network in the future, but with spottier coverage.
In short, if you want AT&T compatibility, be sure to get a phone that supports both 700 and AWS.