How do I downgrade Android 2.1 to 1.6 on a Garminfone? I need to send in my Garminfone to T-Mobile. I've rooted it and installed the 2.1 upgrade. The master reset doesn't restore it to 1.6. I know Super One Click will unroot it. Does anyone know how to revert back to 1.6? Thanks.
not sure about the root process for Garminfone, but if you have access to your bootloader (usually termal like with a list of options) just put a 1.6 stock rom on your sd card and flash it.

I tried googleing "stock 1.6 garminfone rom" with no success, probably because of what GAThrawn said, but someone over at xda-developers might have something if want to go check out there forums.