If you visit the Apple Store, you'll now find an official listing for the Tom Tom for iPhone car kit. As promised, the US$119.95 kit does NOT include the Tom Tom app itself (which will set you back another C-note). Expect delivery in 2-3 weeks, but at least shipping's free.
The main reason to get the mount, apparently, is the additional GPS circuitry it contains to enhance the 3GS's own positioning system. (The jury's also out as to if the kit's circuit would benefit other GPS apps.)
As we've said before, we're not sure if this solution still tops buying a dedicated GPS, which will certainly cost no more (and if you shop wisely, a damned sight less) than the nearly US$300 for the combined hardware/software solution.