Or get the Nuvi 760 from Costco (lifetime satisfaction return policy) for $350 (at least it was on sale last week). Remember to register it online for the free 2009 map udpate.
Its black and matches the bike, at least with my gloves I have no problem using the touchscreen interface (granted I have summer gloves), and has lots of features (more than the zumo):
There are a few problems with the nuvi 760 vs the 550. I looked at the 760 pretty hard before ending up with the zumo. The 760 is a nice GPS but for full fledged moto touring its lacking in a few key areas that are costly to rectify.
1). Not Waterproof (aqua box would work $35)
2). Does not include Ram mount and extra cradle for car mounting. The mount secures the gps ok for a car, but a bike has a better chance of popping it loose. So a ram mount ($40), extra power cable for car/bike ($20), and a good mount ($50-100) for the bik.
3). Nuvi only connects to a bluetooth phone. The zumo can connect to a bluetooth phone and helmet at the same time.
4). Nuvi stores 10 active routes in tracks, where as zumo does 50.
5). Option to select automatic, prompted or no route recalculation.
6). Fancy gadgets like fuel gauge
7). Replaceable battery
8). Slightly better contrast and brightness which is important in the sun.
The nuvi does have a bigger screen. Other than that I don't think it has any big advantages other than cost. If you don't need the durability, bluetooth, and mounting hardware then it's a great option