Final numbers for Los Angeles County voter registration are still a few days away, but the Supervisor Zev Yaroslavsky website has posted a near-final tally. Pulling a few stats from the ZevWeb chart, here's the change in registration between fall 2006 and now.
Decline to state: +169,258
Most of that Democratic surge (to 51.4%) probably occurred during the registration push before the 2008 presidential election. Republican voters are down to 23.6% of the county's total, per ZevWeb. Also this: "The last time Los Angeles County went to the polls for a gubernatorial general election, Bernie Madoff was a respected investment advisor, no one had ever heard of an iPhone and Saddam Hussein was still alive."