Here's a document discussing the exceptions of "at-will" employment.
http://www.bls.gov/opub/mlr/2001/01/art1full.pdf
Specifically of interest is the "Covenant-of-good-faith exception"(only in 11 states, CA being one of them) which I first saw referred to also by someone else here on HN.
At trial, the jury found that Kmart terminated
Ponsock to avoid having to pay him retirement benefits. As
part of his case, he claimed that Kmart’s discharge was in
“bad faith” and that, even without a contract, such a termination gave rise to tort liability. The court agreed, citing the employer-employee relationship as one of the “rare and exceptional cases that the duty [of law] is of such a nature as to give rise to tort liability.”
If I was a Zynga employee I'd definitely be talking to a Labor/Employment lawyer. "Give us back your unvested stock or be fired" sounds pretty "bad faith" to me. Zynga should have just fired the people, but I think they don't want to lose the talent, they'd rather negotiate lower compensation instead of having to find new talent which will most certainly know of Zynga's upcoming IPO & demand proper compensation.