Hands on the Google Nexus One phone
gizmodo.com
gizmodo.com
900/1700/2100 is a pretty common set up for T-Mobile phones. According to FCC documents, this appears to be the only frequencies the Nexus 1 has been tested on.
Really, the only reason this is an issue is because of the rumor Google will be pushing this phone unlocked through retail. I don't think this is true. I think it will be exactly like the G1.
Being that 850/1900 are AT&T's 3G bands, I think you might be confused. I just looked at T-Mobile's and HTC's websites; all of the 3G phones I could find operated on either 2100 or 1700/2100.
Also, see http://en.wikipedia.org/wiki/List_of_UMTS_networks#Americas
All we have is a Google blog entry that specifically says this is a development phone to test new features, and a bunch of speculation.