The entire Garmin 2xxx line from the now discontinued 2610 - the brand new 2730 meet the same waterproof specs of the TomTom and the marine units. The 27xx series share the newer, sun-bright high resolution screens.
The 2610 is what BMW uses as the base unit for the NavII and NavII+ which are have been marketed for motorcycles for years.
The TomTom is definitely on my short list. On paper it fails since it is not compatible with PC software for route planning and/or saving like the Garmins (as per other post), but, IF the GUI is superior that may not be as important to me.
I would love to spend some time with the TomTom Rider, the Garmin 2730 and the Garmin 376c and determine what I want. I have not found a local dealer who has them side-by-side....maybe at a rally??