As for rain, I'll pick up a clear vinyl/PVC bag with drawstrings and put it over the GPS if it shows signs of raining.
Bingo. There is no reason to buy a motorcycle specific GPS, they are way to expensive (2610 exception noted). My Nuvi has 16,000 miles on the bike with no problems. It was $200 a year and a half ago, now you can get one for $109 from radio shack:
refurbed nuvi 350. That was the very first link I found, probably a little research will turn up a cheaper version. Mine had the same one year warranty as a new one, it speaks street names, and you can even get Australian Jill to talk to if you want.
Add in $20 bucks worth for mounting and you have a GPS on your bike for less than $150 shipped. Mine has 16,000 miles on it, plenty of severe weather, works nicely in the car, no problems.