Nowadays it would be shear stupidity if an employer does not have a disclaimer in the employee handbook somewhere to the effect that " . . . this is not to be construed as a contract of employment; . . . ." or ". . . your employment is considered at-will and can be terminated . . . ."
Off-hand, I don't know of any cases that have been WON, which doesn't mean they don't exist. You would have to surf through your District/Circuit court cases or visit a law library.