I ran into this possible battery saving tip while surfing on some Nexus One forum. I never knew you could do this but I did it and was able to change the setting as recommended. I have not noticed any problems with my phone after I did this. I am on T-Mobile. So do this at your own risk.
On the dial pad dial the following set of characters and numbers:
After you dial the final character you will see a menu of settings for the phone. Go to "Phone Information" go down to "preferred network type" and select GSM auto (PRL). Supposedly this will keep the phone from roaming and stay on your carriers network. This is supposed to help the battery.
Let me know if you noticed any battery savings.