Attention, folks who purchased the TomTom car kit for their iPhone 3G or 3GS, only to upgrade to a won't-fit-quite-right iPhone 4: you're in luck. TomTom is now providing a free adapter for the aforementioned car kit that allows an iPhone 4 to fit properly.
Apparently the adapter is simply a piece of plastic that one sticks into place over a spot at the bottom of the dock. It's included in the kit packaging from September 1, though if you bought a car kit before then, you can claim your adapter from TomTom for free.
The TomTom car kit retails for about $120. Obviously, to make full use of the TomTom car kit at all, you'll want to have the $40 iPhone app to go along with it.