I'm not sure I've heard of anyone using a Nuvi on a motorcycle. I don't believe they are waterproof. Their durability on a motorcycle might be less than optimal. Most folks I think are using one of the Garmin StreetPilot series or the new motorcycle specific Garmin Zumo.
There is a forum member who owns a GPS store and he can be very helpful with respect to GPS questions (look in the vendor section of the forum).
Larry