I don't think this is iPhone 3GS Only...
They announced this last night at the WWDC Keynote, but way way way before they announced the 3GS iPhone - it was in with the section about the accessory interface using things like Bluetooth and the Dock connector.
The only 3GS specific things are the video, video editing, voice control and compass. It's typical apple, though - they do the same with the iPod adding new software features on the newer model - the 3G would be able to do the video editing and voice control - they just choose not to.
I don't believe the TomTom is 3GS specific (in fact, they would be bloody stupid to make it so). I think you'll find it's just Version 3 software specific.
To add to your story - TomTom found that the way the GPS works in the 3G isn't brill, but the mount I've seen from TomTom uses the accessory protocol mentioned as part of software version 3 to allow them to embed a much better GPS chip in the cradle, and it docks and charges the device in one go.
As for an A2DP stereo, JVC do a brilliant range of head units, many of the old ones of which are bluetooth compatible using a little £80 module that goes into the multi-changer socket on the back. In fact, you can daisy chain modules, as I have a DAB module, Bluetooth module and iPod module in the back of my KW-AVX800.
Their new range (such as the KD-AVX77) has Bluetooth and Ipod control built in to the head unit now.