This came up in a conversation yesterday, and I tried to use Wolfram Alpha to find the answer (unsuccessfully, sadly). Google provided the following, however:
http://www.calinst.org/bulletins/b1115h.htm
Short answer: 46.9% (!). And check out how much of Nevada is owned by the government for a real shocker.
That’s just what the _federal_ government owns. In California this is Yosemite, Sequoia, Pinnacles. Almost as much (it seems) is owned by the state: most beaches, Big Sur, Big Basin, Henry Coe; plus a good portion by the counties, i.e. Mount Madonna, Almaden Quicksilver. I wonder how much land is actually private, developed or not….