Mark Blumenthal and Charles Franklin have a piece today in Slate Magazine explaining why exit polls in California are meaningless. The short version:

Poll junkies beware: California exit polls are not to be trusted.

California has issued 5.5 million absentee ballots for today’s primary, reaching more than one-third of the 15.7 million total voters registered in the state. As of yesterday, 3 million ballots had already been returned, and state officials expect about 75 percent of the ballots to be returned by the close of polls—that’s 4.125 million people who voted without pulling a lever. (These numbers include both Democratic and Republican ballots.) The remaining ballots are expected to be turned in at polling stations today, just like you drop off a movie rental.

For our purposes, it’s the 3 million ballots that have already been sent back that may play havoc with expectations tonight. Exit polls, as their name implies, measure only the opinions of residents who go to the polls and submit a ballot. If you don’t show up to the voting booth, you’re not going to be part of an exit poll.

The bottom line: given that early voters are suspected to have broken toward Clinton, Obama could be “winning” in an exit poll but lose the election.