My guess would be that TomTom has limited their Car Kit to sole use by their application, but I will inquire with them.
In an interview a manager from TomTom said their adapter can be used by other nav software using all the features of the adapter. I do have the navigon navigator system on my iPhone. On youtube are videos showing that the TomTom adapter works with Navigon and co-pilot even on first Gen iPhone which don't have a GPS.
Here's a video showing Navigon running with TT adapter
http://www.youtube.com/watch?v=71cGSFVwu1E
In different forums I've read that TT will publish SDK on how to incorporate their adapter in other apps.
I do own the TT car kit now.
It works with any GPS based software I tried:
* Navigon Navigator (I find this much better than TomTom.)
* Trails
* iPhone Maps
* Navionics Marine
* GPS MotionX
* Distant Suns
* Star Walk
* and of course iNavX
How did I test it: I drove with my car to a place with bad GPS signal (if I use the iPhone builtin GPS). As soon as I place the iPhone into the TT cradle, the GPS signal appears at once.
Although a message pops up when inserting the iPhone, that there's an app missing (they mean Tom Tom Navigator) but just click on "NO" when asked to connect to the iTunes Store.
Unfortunately you have to say NO everytime you insert your iPhone but the TT cradle works.
I just tested the TomTom car kit for Ipod Touch. Also works with iNavX (not a great surprise). Can't wait to check how it fits at the helm console.