I do it. It is a cheap alternative, but since I use it in the car and always have the map and POI database with me, it works well.
Here is how I do it. I have a Sprint PPC-6600 which is a PDA and phone. I put it in my tankbag in the map window, and it is powered via power in the bag and plugged into my Autocom Active-7 Smart (I had to make my own special cable). The GPS is a bluetooth portable type, and I just put it inside the bag (works great and no issue with interference). I can receive phone calls, make calls (using the Voice Command software), listen to music (I have a 2 GB card in the PDA), navigate with audio, etc. If I want, I just take the whole bag and everything goes.
Specifically, for navigation, I use iGuidance software and a USGlobalSat BT-338 GPS receiver. Its battery lasts about 20 hours of use, but I could power it inside the bag, too. What is really cool is it turns itself off if there is no bluetooth signal. I have the whole maps of FL, GA, NC, and TN on the card.
I would like to find a better way to fix the PDA in the map window, because it does wiggle around a bit, but it works because if it rains it stays dry.
This may not be ideal for everyone, but it works well for me.
-BD