Nielsen has been trialing this technology for about two years: the PeopleMeter (mentioned above by K. Drozdick) is, as far as I know, still undergoing trials in Texas and somewhere in the US midwest.
While the technology is "intrusive", the people ARE volunteers, and the data is considerably more accurate than the set top box and paper log that PeopleMeter replaces. And it does extend accuracy of reporting to venues that are normally impossible to survey without "ground-pounders" (ie, Nielsen employees) watching the people.
Why should we care?
Because, in the current advert-sponsored entertainment environment, anything that provides a more accurate measure of what we REALLY enjoy (as opposed to what the RIAA and MPAA constituents TELL us to enjoy) is going to improve the quality of entertainment content over time. Face it: Coke or Chrysler are only going to sponsor content that gets watched or listened to.
Furthermore, with content segmentation becoming rampant (100 channels of satellite radio, 500 channels of satellite TV, DVDs, CDs - downloads don't count here as they are already tied directly to a customer) knowing what is popular and being enjoyed is much more difficult.
My suggestion to all El Reg regulars out there: go sign up to be a ratings volunteer! Get your idea or what is entertainment registered with the only group that really can influence the content that we get presented with. Take your PeopleMeter to Indy concerts, watch oddball DVDs, listen to pirate mixes in the car - do everything you can to add outlying data points to their database. If enough "intelligent" people start "contaminating" the data, maybe we'll actually see some changes in the entertainment content provider's agenda.
After all, without sponsors, there are no shows. Sony/BMG would understand this quicker than a Congressional subpoena.
Oh, and while you're at it, go sign up for jury duty. Remember, it's supposed to be YOUR PEERS - if you don't go, think of who will be judging YOU when you get busted for writing Open Source or listening to Indy music...
Just me two EuroCents...