Those in CA thinking about implementing such a policy will need to be careful. IANAL, but having been CFO of two CA start ups now, you will need to give CA employees at least 1 day per month worked that will need to be paid upon termination. You can cap the amount of accrued vacation a 10 or more days, but once they've earned a day they can never loose it.
We just switched to this kind of unlimited PTO policy here at VMware. It remains to be seen how it works out but I'm optimistic.