PDI is an incredible source for California voter data and they once again hit it out of the park. They are providing a daily count of VBM returns broken down many ways (e.g. congressional district, county) and by age and ethnic group. They also provide VBM and totals for 2010 and 2012 for comparison. If you want to follow California elections, check this site every morning.
I'm not going to get into an analysis of the VBM returns at this point. You have about 2% of the 2010 VBM totals right now and that's not representative enough to tell us anything. What's more, some counties are quick to report while others are slow. So a Republican part of a district might've reported, while a Democratic part might not. That'd skew results. Sacramento is usually not quick and they've reported 13 VBM ballots returned for CA-7. Yes, 13.
Once the returns start becoming significant the numbers to watch are VBM party breakdown for 2012 and 2014. I don't have any evidence there'll be a strong correlation between VBM party breakdown and results across years and we saw earlier this year that increased Democratic VBM cannibalized some of their election day voting. Still, if a party is doing better with VBM than 2012, they are likely to do better in the final vote.