Founder's can cash out without screwing employees. I had almost no stock as an employee in my first startup, but the founders took care of me post-acquisition.
That kind of thing happens a lot, but you shouldn't count on anything not in writing (and probably not even then, unless it's on a cashier's check).
Google's founders famously helped an early employee (Scott Hassan) who had done a lot of work and didn't have much on-paper equity. He's now running an awesome robotics startup, Willow Garage.
