Will it spread?  Without a doubt.  California is the first to legalize primarily because our state constitution allows for direct initiatives.  The other benefit of this is that the law can be written by non-politicians, which is why the medical marijuana rules, for example, were so vague that local municipalities were able to interpret them without law-and-order types interfering at the state regulatory level.  

The reason it will spread quickly is revenue.  As soon as California legalizes there will be an immediate set of industries that pop-up around it.  If these companies get a head start they will be able to dominate the marijuana business.  It is in the best interest of say, Washington and Oregon, to legalize for business competition reasons.

Also, the citizens of various states will be traveling to California to fill up our tax coffers instead of spending that disposable income in their local economies.  

There is data supporting all of this because prohibition has happened before and also because it still exists in some counties and states to an extent.  The main difference is that marijuana does not have the severe social issues that alcohol comes with.

Naturally, alcohol and cigarette interests (along with the pharmaceutical companies) are the biggest opponents to legalized marijuana.  It's business and they're the competition.

Marijuana legalization will be on the November ballot in California and will definitely be the most interesting thing to happen in this election in the US.  There is a huge underground economy and industry already in place that will spring into action faster than most people can believe.  The cultural impact will also be enormous.