That sounds like it would do the trick. I didn't run into any such products while searching, but I probably wasn't using he right terms for it.

Here's a device that attaches to the steering wheel: http://www.dealextreme.com/details.dx/sku.28835

You'd still need to connect something else for audio, the above is a remote only. I suppose using the headphone output would still work. Or, if hacking is desired, I'm sure it would be a lot easier to hack audio outputs from the dock connector on the TomTom mount than it would be to merge it with that Parrot device. Provided there are still pins in the dock connector used for analog audio.

But here comes Kensington again: http://www.amazon.com/Kensington-LiquidAUX-Bluetooth-Remote-Phones/dp/B0011UIX2K

Another LiquidAUX product, this one all about BlueTooth, with the same steering wheel mounted remote as the previous one I linked the other day. The first potential caveat to check is to make sure the current iPhone OS supports all the required commands over BT, including Next/Previous tracks. Apparently version 3.0 of the software didn't.

The Kensington solution is even cheaper than the first product from DealExtreme. At least on Amazon right now (at $29 you save $70 over "regular" price). But it doesn't have a display on the steering wheel to show caller ID. Of course the benefit is that the remote is nice and small, as opposed to the somewhat hulking mass of the first link.