A total of 1144 or so are needed to secure the nomination. A fair number of states, including big ones like Texas and New York, have proportional distribution of delegates. So it may still not be easy for Romney to get to the total he needs, even if he wins a lot of states. Usually, our June primary is so late, it hardly matters, but it could be interesting. California is a winner take all state, with 169 delegates. That's (slightly) more than all of the remaining delegates to be voted on after California.
I don't know how likely the scenario is -- 538 notes that Romney so far is skating right along the 'majority track' that would lead him to victory right at the end, a pattern which definitely ups the odds of California relevance if it continues. It would be interesting if we get to June and our 169 delegates would (barely) put Romney over the top. I find it hard to believe that Santorum has narrowed Romney's lead in California to 6%, but such is what a recent Field Poll shows.
California could be the battleground for all of the anti-Romney forces to come together to make their last stand to prevent him from getting the delegates he needs.
A similar scenario raises the really amusing spectre of Romney getting the delegates he needs in the absolutely very last state in the union to hold its primaries... winner-take-all Utah.