New from the Center for Inclusive Democracy (and democracy all-star Mindy Romero will explain it to you at greater length, here).  Unsurprisingly, registration at California DMVs has become the most frequent means of registration since the process went opt-out (you answer eligibility questions in the process of completing a transaction and you’re offered a chance to opt out of registration; if you’re eligible, you get registered at the end of the DMV transaction unless you specifically opt out).  It’s far more likely to get used than an opt-in system.  It’s also, as the report points out, another way to make sure that the rolls are accurate and up to date, with the proper address for voters who may have moved. 

The report points out that there’s still a turnout gap (exacerbated for younger voters) between people who register at the DMV and people who affirmatively seek out options to register online: registration isn’t the same thing as mobilization.  But opt-out registration at the DMV and registration online aren’t likely to reach precisely the same audience, either, so the growth of DMV registration isn’t just a substitution effect.  (For very sound methodological reasons, the report doesn’t attempt to make a claim about the overall impact of opt-out DMV registration on turnout.)