In the 2004 election, 360,000 San Francisco voters, around 75% of registered, voted.
2008 was predicted to have similar or higher turnout in SF. However, San Francisco ended up with only 50% turnout, 240,000 San Francisco voters! 1/3 less than in 2004.
In comparison, turnout was 64% for Santa Clara county, 55% in San Mateo county, 69% in Santa Cruz county, 63% in LA, 66% in San Diego. Nationally turnout was 64% and in California it was 60%, far below estimates of 70-80% that the Field Poll predicted.
What happened? I suspect that many Californians thought that the election (esp. Obama v McCain) was already won, and didn't bother to vote. This sort of mentality is more common the younger the voter, and the younger the voter, the more likely the were to vote No. The greatest amount of blame for Prop 8's passage should lie with the people who opposed Prop 8 but didn't bother to show up to the polls. I couldn't find a single story in the media or blogs mentioning how poor turnout in CA turned out to be.
Ironically, if Obama had run a closer race, I think Prop 8 would have been rejected.