A New York Times environment blog article (with many, many links) comes down firmly on the side of "natural variability" for the cause of the current drought in CA. The main person quoted is a climate scientist who does NOT cast doubt on climate change itself, simply on that being the cause of the current CA drought.
I personally have no doubt that climate change is real and is caused by human activity. I also am strongly opposed to blaming climate change every time the weather acts a bit less than humans would like in a particular region. That is more likely to discredit the concept of human induced climate change - as well as the individual "who cries wolf". The issue is too serious to be thrown around so casually.
My reading of "The Atlantic" article below is that the grand CA water project Brown proposes is 50% over-engineering and that the real solution consists of a lot of 'little' local efforts at conservation, desalinization, gray-water, more local storage, greater efficiency, etc. In addition, it appears that CA agribusiness would not suffer all that greatly (about 6%) under one computer model of climate change in CA.
The worst CA drought known was in 1600-1650 when the average was 20% of normal rainfall.