My comments and summaries:
GPS Cost: No mention of ATT charging for access to its server assistance as yet. As long as the aGPS is of the handset based type, it can operate without the assistance if need be.
Mistakes following GPS maps: already happens a lot. Trucks turning down roads with limited bridges. Many people turn up the wrong way on four-lane highways, because they simply followed the "turn left" instruction. Users need to be aware.
Navigation vs Google: Real navigation has a windshield view, optional voice instructions, and the ability to recalculate on the fly. Google's turn-by-turn instructions are just a list. My personal guess is that Google's contracts with their map providers prevent Google from doing a realtime navigation app.
Navigation app cost: As mentioned before, $100-$150 for the software and a country's database is the usual for PDAs and smartphones.
Navigation multitasking: My question still stands... what will happen if a call comes in while you're using a navigation program? Apple's rules say it must exit. That would make it rough to use, compared to other phones that continue the nav.
That said, O2 Germany is supposedly going to include TomTom 7 with the new HTC Diamond. Reports say TomTom is tired of piracy and would rather make OEM deals. Alas, Apple hasn't even let TomTom into the developer program yet.