Can't believe no one has mentioned the old Garmin Streetpilot series (2610/2620/2720/2820, etc). These were Garmin's motorcycle gps's before the Zumo's came out. Garmin even sells a hardwired connector and handlebar mount.
They're rugged, waterproof, bomb proof, and best of all cheap! A 2720 can be found on ebay/craigslist for around $50 if you look around. Ram mount is around $20 and the Garmin power cable is around the same. $100 and you have an awesome, fully functional, motorcycle gps.
Trust me when I say you're not missing any features compared to the newer Garmin's. These have all the features you would want; uploadable routes, custom POI's, track logging, always on compass (big deal for me), latest maps when updated. In fact, I actually like the UI a lot more on these compared to the newer Garmin's. The new one's just feel way too dumbed down.
A few years back, my 2720 died on a roadtrip. I ended up ordering a Zumo 660 to use. The 660, despite costing ~$750, offered zero real additional features compared to my prehistoric 2720. The main advantages are the brighter, bigger screen. Otherwise, it didn't do anything that my 2720 couldn't. When I got home, I sold the 660, and bought a 2820. When the 2820 died, I bought another 2720.
It'll be a sad day when I can't get another Streetpilot