Colorado athletic director Mike Bohn might have been promised a North/South split when he was invited to the conference, putting him in a conference with USC/UCLA and Arizona/ASU. Apparently their largest alumni base outside of the Rocky Mountain state is southern California. This also seems to be the alignment most of the conference administrators seem to favor the North/South Division.

Personally, I'm a bigger fan of it because it keeps travel costs down a bit and the Cougs will still be in at least the Bay Area every year and southern California probably every other year. 

Link to the story on Ted Miller's Pac-10 Blog. Pac-10 Alignment