I've got a similar setup to Riktus - iPaq 2210 with CF GPS receiver, and TomTom 5. It works fine in the car, getting me from A to B, so I tried mounting it on the GS using a RAM cradle. I used a ciggie-lighter type of power lead with an adaptor for the BMW accs socket. I connected the audio out via an Autocom, but had problems with interference, so in the end just plugged a small headphone directly into the iPaq. I didn't worry at this stage about waterproofing the setup.
The big problem I found was viewing the screen of the PDA - it was just too reflective, and even with the backlight on it was difficult to see. The voice directions could be heard OK, but I found it a real pain to have a lead hanging from my helmet - but thats just a personal hangup, lots of others don't mind this.
Apart from the screen reflections the other drawback is that the TomTom s/w doesn't provide a way of plotting a route on a PC and uploading it to the PDA. In the end I was lucky enough to get one of the bargain Quest 1s from Halfords, which is a much better solution for a motorcycling - but crap compared to the TomTom for use in the car (no post code entry in Quest, no speed camera warnings etc etc.) BTW, I don't use the voice instructions from the Quest - just glance at the screen.
My advice

- if you've already got a PDA think carefully about how you mount it and whether you will really see the screen. Don't spend a heap of money on waterproof boxes, audio leads etc. until you're convinced that it does what you want it to. If not - buy a bike oriented dedicated GPS
HTH