I've seen a number of moto riders on the road using PDA based GPSRs. Although they are generally not water and dust proof, they seem to work OK.
What you really have to do 'due diligence' for is the readability of the PDA screen in bright sunlight, and your ability to operate the controls of the PDA when you are on the moto. Some PDAs have great contrast in sunlight, others are hopeless. Some have big buttons, some require that you use a stylus (forget about that on a moto). This is not only a hardware issue, it is a software issue as well - how have the developers of the application designed the user interface?
My own take on PDA based GPS is this: If you already own a good quality PDA and you use it a lot (meaning, you are really familiar with how it operates), then, you might like having the GPS and PDA in one unit, because it will be less stuff to carry, and you have all your data in front of you. But, if you don't own a PDA, then don't even consider using PDA based navigation on a motorcycle. It would be like trying to change a head gasket with a Swiss army knife - it could be done, but would be a real PITA compared with using the correct tools for the job.
Michael