I don't have a TV module, but I see no reason that a standard digital tuner wouldn't work and you could hook it up to the video inputs... but as I understand it, the digital signals in the US are such that they do not work very well while in motion, you would likely only get good reception while you were not moving... as you appear to be in the UK, you may have better luck, there are some 3rd party vendors which supply plug and play solutions.
I have also seen some nice mobile satellite setups but you'd need a rather ugly antenna for that.
BMW produced a hybrid (analogue & digital) TV tuner for the 3 & 5 series & X5 which is a plug & play replacement for the pre-05 L322 analogue unit (this is the silver box in the boot and has nothing to do with the screen in the dashboard which is just a monitor). They're fairly rare though, so when they occasionally appear on eBay they typically sell for £400-£500.
Using the Aux Video is a good option, coupled with an off the shelf DTV ATSC Tuner module. It's a shame that the units do not have RGB or S-video inputs that would allow a better signal quality to the monitor. Then again the screen size is small enough that HDTV signals would still look pretty decent on the 7 inch screen.