I know that quite a few people use the TomTom kit in addition to a io-play unit.

The TomTom cradle only uses bluetooth for audio, the GPS data goes over the dock connector, so you can quite happily use both at the same time.

While I can't comment on the ioplay unit as I don't have one. I do have the TomTom mount and it's excellent.

Edit:

Humn, on further inspection you need to plug a dongle into the dock connector to stream music, without it you can only use it handsfree. Not much use really.