This helps John McCain because those conservative voters may not have come out in great numbers for him, but they will come out now to vote for this amendment, and they are more likely to vote for McCain than for the Democrat once they are already voting. That's not to say that California will go red, but it is to say that the Democratic nominee will have to devote more resources to this very expensive to campaign in state.
And I admit, this was my first thought too.
But thinking about it more, this might actually prove dangerous for McCain and his cash-strapped campaign. California is, as was mentioned, a very expensive state to campaign in. Under normal circumstances, it wouldn't surprise me to see McCain write it off entirely as a waste of resources. The gay marriage ruling might mobilize conservatives enough so as to convince McCain to make a play there. But if he does, he'll have to devote resources too -- and he has far less to spare than the Democrats do.
I'm reminded of Republicans investing in the 2006 New Jersey Senate race in a year where they also had their backs against the wall. Like California, New Jersey is pretty blue and very expensive to campaign in. Republicans were convinced that Robert Menendez was a beatable Democratic candidate though, so they decided to contest it. They ended up losing (just as they still likely will in California), and the resources they put into the Garden State would have been invaluable in states that were objectively much closer fights: Virginia, Missouri, and South Dakota. Had they not been tempted by New Jersey, they might well still have control of the Senate today. Similarly, if the gay marriage ruling tempts McCain to make a play for California, he might regret if it draws desperately needed campaign resources away from true battleground states. Democrats can afford to play the 50-state field -- Republicans this year can't.