Over on CaliforniaOpenCarry.org, there is a memo from the Los Angeles Sheriff's Department (http://www.californiaopencarry.org/m...SD_oc_memo.pdf) and in it they write:

"High Capacity Magazines (more than 10 rounds): Effective January 1, 2000, it became a felony to manufacture, import, sell, keep for sale, give, or lend,
a high-capacity magazine. There is no prohibition on the simple possession of a high capacity magazine." (Italics added for emphasis)

So you can possess high capacity magazines in California according to this memo.