I just gave my wife a Nuvi 760 for Christmas. I took it out of the box before wrapping it to charge it up and load in some waypoints, pictures, and MP3s. It was so impressive that I was immediately jealous. A motorcycle-friendly version of the Nuvi would be excellent. Maybe the Zumo is a little like that. I'm not familiar with it and I probably won't be any time soon. GPSs are not cheap dates.
I would worry about the lack of waterproofing, keeping it attached, and maybe vibration if putting it on a motorcycle. One solution for vibration is to do something like RKA
put together. They will configure their tankbags with a RAM mount and wiring for GPS and comm devices. You could probably put a RAM ball on most any tank bag by this method.
The Nuvi uses a nice clip-in mount/interface with a Garmin propriatary ball mount. There is a RAM adapter
to fit this ball mount. You would bolt the adapter into a standard diamond base RAM ball and then it can be attached to whatever RAM mount you like. Unfortunately, I don't think the Garmin clip in mount is strong enough. Its OK for a car, but it seems to me like a good bump on a motorcycle might pop it out, either the Nuvi from the cradle or the Garmin ball mount itself. You might be able to do something with 3M Dual Lock
. If you haven't used Dual Lock fasteners, you should really check it out. It is what Velco becomes when it grows up. Very strong, very secure and doesn't wear out. It is expensive but worth it. I get it by the foot at a local electronics supply. I think they stick linemen to power poles with it.
Another possibility is the RAM AquaBox (large)
. It is waterproof, and might help with vibration if you could pack it with the right kind of foam. If you put the Nuvi on the docking cradle then in, the power/interface cable connects in a way that would probably fit through the cable port in the AquaBox. I haven't tried this, but I have been thinking about it.
I'd be interested to hear what you come up with.