If you are one of those people who watches most of your television time-shifted using your DVR, then you'll be interested in this news. For the first time in history, Nielsen Media Research - the organization relied upon by the TV networks and advertisers will be using a new ratings system called "live-plus-three-day commercial rating" that is changing Nielsen's methods in two ways. First, it measures TV viewership for commercial breaks instead of the entire TV program. Secondly, Nielsen will include TV viewership of a show over a three-day time period. This second change means if you time-shift TV shows and watch a show up to three days after it airs it will be counted as being viewed. In the past only those shows watched during the original airing date were counted.
Note that Nielsen has test families that have Nielsen devices to do the measurement so they aren't really counting what you watch of course (unless you have a Nielsen measuring box), but it does mean more of those shows in popular time-slots will be measured because a show that goes head-to-head with another popular show will likely be watched time-shifted and will now be counted.
Wondering how Nielsen will identify those TV commercials to measure them? Well the TV Networks will transmit an audio code (inaudible to our ears) any time there is a break. Currently, the Home Theater PC software I use automatically detects the start and end of commercials by detecting changes such as the fade to black before a commercial, fast scene transitions and TV logo disappearing. Beyond TV does this automatically and with the other HTPC software programs such as SageTV, MythTV, MediaPortal, GBPVR and Media Center you can get an add-on to do the same thing. If the HTPC software makers could incorporate audio detection techniques into their commercial detection they could likely make them even more accurate.
It will be interesting to see how the changes in Nielsen ratings affect the ratings of various shows this Fall TV season.