Imagine getting a free cell phone, but instead of being forced to listen to advertising in exchange for your free-ride, it listens to you instead. That's right. Every so often, it records the activity going on around you and then uses a computer algorithm to create a "signature" from those sounds passes that signature back to a processing center, where another algorithm compares that signature to its master database to recognize what media type and "program" you are listening to - be it a music CD, TV/radio program, Muzak, rock concert, or whatever. Your data is then compiled with many others' to create media ratings. By only passing a signature and not the raw audio itself, no actual conversations get eavesdropped on - which would be illegal in many places.
In this age of TiVO, iPod, and other time- and/or location-shifting devices to allow viewers and listeners to consume media when and where it is convenient, traditional ways of measuring and estimating ratings are not as effective - hence the market's experimentation with this type of technology.
If you are interested in reading more, one company that is selling such a service is IMMI (Integrated Media Measurement Inc). There is a link on their home page to a Wall Street Journal review of several companies doing similar things.
(Hat Tip: Bruce Schneier's CRYPTO-GRAM email newsletter, January 17, 2007 edition)