The new ratings will be gathered the same way Nielsen compiles its existing TV ratings. The firm will use set-top monitors in 10,000 Nielsen homes, checking viewers' TVs several times per minute to determine what show is on. The monitors record whether viewers are changing the channel during a commercial break or zapping through the break with a DVR.
As for visits to the fridge or bathroom (or presumably both - preferably not at the same time), Nielsen viewers are supposed to use another device to record when they leave a room in which a set is playing. The system isn't foolproof, but Nielsen says the company believes more than 90% of its viewers comply with this rule.
Proving that viewers will do almost anything for a few extra bucks. As long as they don't confuse the device with toilet paper I guess all's OK!
On a more serious note, the Nielsen commercials ratings are almost guaranteed to trigger another wave of shifting ad dollars to the Internet and more measurable/pay as you go mechanisms. TV networks in turn will be forced to embrace Internet TV and digital interactivity ever more rapidly. Poor luvvies!