Keef,
If your using Word/Excel etc I wouldn't recommend getting an iPhone. An XDA or something similar that runs Windows Mobile software would be a better option. The problem with the iPhone is that it runs on UNIX where as the programs you use run on Windows, thats why they dont work very well but give it time.
Yeh there are a lot of assumptions on the iPhone, we get things like the "iPhone doesn't have a sim", "iPhone has a special SIM" etc. in reality it's just a normal SIM card with extra access points on the network very easy to setup.
You can use the iPhone as a modem with certain software HOWEVER I would highly recommend you don't even try, for two reasons. First of all the phones warranty may be affected and second of all the internet packages provided for all mobile phones, regardless of the network don't allow you to connect using a laptop thats why Mobile Broadband is there. We catch customers out on a regular basis trying to use the phones as modems, they aren't designed for that.
In terms of the GPS, there were rumours that TomTom were going to create a GPS addon for the phone however nothing has came to light yet. We have been told by Apple that it can do that however many believe that the signal strenght isn't powerful enough.
I'm not sure which si correct, but I don't understand why the use a 2d map GPS rather than a 3d GPS software like TomTom.