Well, it only took a few months, and now the TomTom for iPhone app us out. It'll cost you $99 and it's a 1.2G download. No word on the accompanying car kit yet.
Update: Looks like the holder will do some hooking-in-GPS magic and will work with 3rd party apps, so you'll be able to buy the Navigon, iGo, etc and have it get the enhanced GPS from the TomTom car kit. Of course, to me this also means that the car kit will probably cost big $$, and when you can get a brand new TomTom GPS for $149CND..... Looks like they're either pricing themselves out of the market or relying on the fact that people will pay for the convenience of having everything on the iPhone :)
Also this means that iPod Touch users will be able to use it, which brings in a whole new level of interesting....