Good news: According to Yann Lafargue, PR guy at TomTom, the incoming TomTom GPS Car Kit will work with the iPod touch, enabling full GPS capabilities in Apple's smart multimedia player:

Oui, je confirme et il fonctionnera avec un iPod Touch ou d'autres logiciels de navigations concurrents. Pour la disponibilité, je ne peux pas répondre.

It will work with the iPod touch and other competing GPS (iPhone OS) software. I can't comment on availability.

In other words, the car kit contains a full GPS, which apparently will not only work with the iPod touch, but also replace the iPhone's GPS with a faster, more accurate unit. [Mac4ever via Macenstein]