It is the trend everywhere. It is a late, and IMO pretty amazing, trend in CA.
At my usual range the ratio for rifles is about 50% AR (including .308 versions), 25% combloc semi-auto ("AK"s), 10% 10/22s, 10% bolt action hunting rifles, and 5% everything else (lever guns, bullpups, single shots, misc. other .22s, etc).
Before I left California the state of ARs there was bleak and marginal. I never saw them when I went shooting. There were a few FFLs who were willing to transfer "Off-list Lowers", and a whole bunch of hoop jumping and wishful thinking. Guys were bolting magazines in place and popping the rear pin to reload. Guys were buying dozens of off list lowers in the hopes that CA would ban them, allowing the owner to register them all as California Assault Weapons so they could have pistol grips and detachable magazines at the same time. Guys were designing lower receivers that didn't have pistol grips but would take a standard AR upper.
I visited a California gun store in December of 2006 and there wasn't a single "black rifle" in the place. The most evil looking gun was a Remington 870 shotgun with synthetic stock and pistol grip.
In January of 2007, at about the same time I was pulling a moving trailer out of the state towards my new home in Texas, the first of the modern AR "bullet button" setups was being finalized. This is something that had been talked about on calguns.net for a few years but someone made it happen.
Fast forward to last month. I was visiting California and stopped in at that same gun store I had last seen in 2006. Not to buy anything, but out of macabre curiosity after years of being able to buy just about anything with far fewer silly laws to hold me back. What did I see?
At least half their inventory was AR-pattern rifles. They had 5.56 and 7.62, they had some 6.5 models. They had a wall with various AR magazines (all 10rd of course). Same store, total change. From a distance it looked like the first gun store I visited after moving to TX in 2007. Of course they were all set up with bullet buttons, but it was really amazing to see the results of 7 years positive effort.
I didn't go to a shooting range during my visit. BLM land in the Mojave, yes, but nowhere I had to shoot within sight of other humans. I almost wish I had. I bet I would have been pleasantly surprised at the number of AR type rifles.
Anyway, to answer the question: they are extremely common in most of the US (by land area), and California is playing delayed catch-up with a huge trend...which I suspect bugs a lot of California polits no end.
That is one trip I don't take any guns on. 
I never thought I'd like it THAT much! 
