CA's at-will is a little more complex than that, due to the implied good faith covenant.

One (not primary, but fairly clear) source:

http://www.cslaw.com/our-practice/san-francisco/employment-c...

Googling "california at-will good faith" will unearth much more.

Edit: more detail, including that this isn't specific to explicit employment contracts. Also, I'm in no way saying this case would fall under this, only that at-will isn't absolute in CA, even beyond the normal discrimination boundaries.

http://legal-dictionary.thefreedictionary.com/Breach+of+an+I...