Google has announced that Nexus One will now be available for AT&T in US and Rogers Wireless in Canada. When Nexus One was launched, it was only available on the Google Web Store, unlocked version so users can user any GSM network.
However, different carriers have different 3G frequencies. Nexus One in the start worked with almost all the carriers except AT&T and Rogers because of its 3G frequency. So users using any of these SIM cards usually could access 2G or EDGE network.