On the Nexus One it is possible to bring up a Field Test mode using the following code: *#*#4636#*#*

This will bring up four sections of Test information that you can switch to;
Phone information
Battery information
Battery history
Usage statistics

There may be other codes that give even more information, particularly about the local cell towers you are connected to. I have not found these yet, but will do if/when I do.

If you know of any other Nexus One codes please post them here.