It depends on where in California you live since each county has different laws. I would think your biggest obstacle is getting that caregiver grower status, not the Feds. Everyone would love to legally grow but I imagine it's pretty tought o actually get to that point. I would get in touch with WAMM ( http://www.wamm.org/ ) they'd probably be able to steer you in the right direction.

As far as the Feds are concerned, seedbare is right, the Feds don't care about California law and can bust growers if they want to. I think a lot depends on how much you intend to grow, however. If, for example, you have a sick family member or friend and just want to grow a few plants to provide for them you're probably ok. They could still legally bust you but why bother? If, on the other hand, you rent out a warehouse somewhere and are growing a few thousand plants that's a different story. From what I have read, the feds really aren't too concerned with California medical growers unless they are tied to organized crime. They recently busted a few medical cannabis clubs in San Francisco and everyone thought it was the beginning of the end. Apparently these clubs had clear ties to organized crime, however, and they were only a few clubs busted out of a hundred or more now in the city.