This is probably one of the more pointless things i've done out of boredom.
Since you are messing around with system files, do a backup to be safe
Download and open root explorer go to /system, mount r/w, hold down build.prop, and select open in text editor
In there, i changed
(its all in the top rows)
ro.build.id=ESE81 to ro.build.id=FRF50
ro.build.display.id=ESE81 to ro.build.display.id=FRF50
ro.build.version.release=2.1-update1 to ro.build.version.release=2.2
ro.product.model=Droid to ro.product.model=Nexus One
then reboot. Go to settings, about phone, and look @ firmware version and Build Number
For some custumization fun, change it to what you want to show..
To change it back, go back into root explorer, /system, mount r/w, hold down build.prop, delete, hold down build.prop.back, and rename to build.prop then reset for it to change back