Time Yeung at the PERB Blog has an excellent summary of the California Supreme Court's decision on the legality of the Governor's furloughs.
Tim's article can be read here.
Short answer - the governor does not have the authority to impose mandatory furloughs. Only the legislature may take such action because the legislature is the body which ultimately determines the terms and conditions of employment. However, because the legislature ratified the Governor's decisions later, the employees are not entitled to relief. This is essentially a Chick Hearn special - "no harm, no foul."