So it looks like the Nexus has a coverage problem on the T-Mobile network, and there is a fix going out to help with that. I wonder if our push is being held to include this patch for the Nexus guys, so we are all on the same code line.
Since the Nexus One is currently GSM only and would use a different radio, this could be a GSM only problem. Or even a problem with the Nexus One's radio (there's a firmware for that too). It could also be a problem created by an HTC patch to 2.1 (read not in the generic 2.1 code).
If I had to guess, it is probably a problem with the radio. The reason I say that is that before the Droid I had an HTC Touch Pro and the radio on it was pretty bad. There were many discussions on which revision was better, but in the end, it was pretty obvious that HTC didn't make the radio a priority.