Given a nexus one (rather any Android phone), is it a way to find out which 3g band it supports (t-mobile or att), without inserting any SIM cards ? currently I'm just interested in nexus one, but generic info can be useful too.
Yes there is. Take off the back cover off the Nexus One, remove the battery and read the FCC ID number at the top of the black area.
The FCC # for the AT&T version is NM8PB99110
The FCC # for the T-Mobile version is NM8PB99100