From the TomTom FAQs (see #14):
14. When will the car kit be available?
The TomTom car kit availability was originally planned for this summer. But we have decided to take some extra weeks in order to deliver the highest quality on this innovative product. So, the car kit will become available this October on http://www.tomtom.com.
Register your email address at iphone.tomtom.com and you will be one of the first to know when it is available to buy.