This is supported by the fact that my first-gen iPhone (2G) fits into my TomTom kit fine, and this phone is bulkier around the top/bottom where it fits into the mount.
I think the car kit is built with enough room for these kind of changes. I used to use my iphone with a hard case and it would fit in the carkit just fine..
I've been wondering that myself. 30 dollar a pop cases, docks, car kits... I only have a couple hundred wrapped up in that crap, and it might all become useless. I know a lot of other people spend much more!
I believe that Tom Tom will consider that the iPhone is a premium mobile device and as such provide superior customer support. In the event the dimensions of the newer iPhone are significantly different they should make it right. I would expect them to provide a "fix" at the very least.