I think you have all missed a key part of the OP's post. He would be buying a docking station that includes a GPS receiver. This allows the use of the device without having to rely on a data connection.
The docking station will give him his position via GPS, but unless you have a LOT of memory to load the entire map in, you will run out of map and into gray space. I don't believe the phone will get the map data via the GPS input. Since I have not tried using a phone with a GPS dock, I could be wrong. I used a GPS enabled smart phone and Google maps. In addition to the map problem the phone sucks power at a HUGE rate with the GPS receiver turned on. It ate my battery in a little over two hours.
I've learned a bunch here so far, and found out a little more searching today on the WWW. Several posters appear correct that the cheaper nav apps (MotionX for example) need an active data connection to update maps, whereas Navigon (seems to be the preferred app) loads the entire map into the unit from the start.
As also correctly stated, the iTouch does NOT have the GPS receiver included, but the listed docks do have the receiver included. Also, these docks have cigarette lighter-type power connections so I should be able to keep my GPS and music powered up continually.
You all have brought up the issues of rain and vibrations which make me more nervous. I think I stumbled across some kind of waterproof enclosure (like for diving with cameras I guess) but don't know how compatible it might be with specific units.
However it turns out, I'll give some kind of update......
thanks