We have a Garmin Nuvi 265t. It's currently not mounted on the bike but we're seriously looking at the
Powerlet system. They have a mount for it that works with their power outlet system. One huge advantage is that it comes with a short power cable so you don't have to worry about what to do with the 3-4' cable that comes with the Garmin.
Who else has one? What kind is it? How did you mount it?