Today, the Pac-10 officially no longer exists, and the Pac-12 does. In the states of Utah and Colorado, it's even an "official" "day", complete with a declaration by the states' respective Governors.
For the first time since 1978, when the Pac-8 became the Pac-10, the Conference makeup has changed, and though nothing much has actually changed today, other than the fact that you can now go to either www.pac-12.org OR www.pac-10.org and get to the same place, because the changes and the coverage have actually been going on for months now, it is still the "official" day when things move on.
Welcome Colorado and Utah, and welcome "The Ralphie Report" and "Block U"!