http://www.wireless.att.com/cell-phone-service/legal/plan-terms.jsp
"The parties agree that AT&T has the right to impose additional charges if you use more than 5 gigabytes in a month. Prior to the imposition of any additional charges, AT&T shall provide you with notice and you shall have the right to terminate your service."
Rogers 6GB > AT&T's 5GB soft-cap.
AT&T IS NOT UNLIMITED.