According to MacNN, Apple has been using shell companies - small corporate entities, in this case controlled by Apple - to file for trademarks that may be used as operating system names.
Image courtesy of Mark J. Miller
Because OS X has been using names and version numbers since it was first released, we can assume that it's the target for such recent trademark filings as:
Yosemite, Redwood, Mammoth
California, Big Sur, Pacific
MacRumors discovered additional trademark filings:
Diablo, Miramar, Rincon, El Cap
Farallon, Tiburon, Monterey
Skyline, Shasta, Sierra
While we're pretty sure that OS X is in line for some of these names, it's entirely possible that Apple will also give iOS a public moniker, one more interesting than just its current version number.
Looking at the list of names, it strikes me that some names are related to the California coast, and some are related to California's various mountainous areas. If we group the names accordingly, the list would look like this:
Coastal names: Big Sur, Pacific, Miramar, Rincon, Farallon, Tiburon, and Monterey.
Mountain region names: Yosemite, Redwood, Diablo, Skyline, Shasta, Sierra.
California as a product name could go into either group, although just for balance, I think it belongs in the mountain group.
So, will the coastal names be used for OS X, and the mountain names for iOS? We'll find out sometime this summer.