Google posted a short announcement via their Nexus One blog that:
Starting today, an additional version of the Nexus One is available from the Google web store that is compatible with AT&T’s 3G network. This new model can be purchased as an unlocked device without a service plan. In addition to AT&T’s 3G network, this device will also run on Rogers Wireless in Canada. And like the first version of the Nexus One, it can be used with most GSM operators globally.
Additionally, Nexus One devices can now be shipped to Canada from Google’s web store, and will work with a SIM from Rogers Wireless. To find out which version you should purchase, based on your mobile service provider, please visit our Help Center.
A screenshot of the purchasing options on google.com/phone/ can be found below: