Here is some information from Wikipedia about the US version of the N1.
Nexus One - Wikipedia, the free encyclopedia
T-Mobile Version US - PB99100
AT&T Version US - PB99110
Carriers T-Mobile USA, AT&T Wireless, et al.
Compatible networks GSM/EDGE 850/900/1800/1900
UMTS 850/1900/2100 (AT&T Bands)
UMTS 900/AWS/2100 (T-Mobile USA Bands)
HSDPA 7.2 Mbps
HSUPA 2 Mbps
GPRS Class 10
As of March 16, 2010, there are two versions of the Nexus One. Both versions of the Nexus One cover most major GSM and 3G providers in the US, Europe and Asia. On both phones, the GSM radio frequencies covered are 850 MHz, 900 MHz, 1800 MHz, and 1900 MHz. The original Nexus One (PB99100) also came along with UMTS frequency bands 1 (2100 MHz), 4 (1700 MHz), and 8 (900 MHz), allowing it to access 3G throughout Europe, as well as on T-Mobile USA and WIND Mobile and Mobilicity and Videotron in Canada. The second version of the Nexus One (PB99110) supports UMTS frequency bands 1 (2100 MHz), 2 (1900 MHz), and 5 (850 MHz), allowing it to access 3G in most of Europe, as well as on AT&T Mobility in the USA and Rogers Wireless, Fido Solutions, Bell Mobility, and Telus Mobility in Canada. The second version of the Nexus One also works on Telecom New Zealand's XT Mobile Network, Telcel's 3G Network and Telstra's Next-G network.
The UMTS radio supports High Speed Packet Access; HSDPA at 7.2 Mbit/s, and HSUPA at 2 Mbit/s.
For more information on how to identify each model of the Nexus One and the differences between the two, please see this discussion on the official Android forum.