Having been through this very scenario, you'd all be wise to go back and read your stock option grants very carefully because odds are very good that the fine print says that the company can 'buy' any unvested options back for the strike price, meaning you don't get squat. At a certain advertising company (that I'm perfectly happy to name - fastclick, taken over by valueclick), the acquiring company turned around and revoked the unvested options of every non-executive employee at the acquired company. They turned that around and offered a much less lucrative deal to anyone that stayed, then promptly drove the company into the ground - or should I say, continued the process of driving the company into the ground.
IMO, the kind of people who would do something like this tend to be in the process of making their own bed. This'll come back to haunt Zynga, one way or another. The whole point of offering equity is because early employees are taking a huge risk, precisely because the potential upside is large. To have the efforts of those employees drive the value of the company up to the point that the company then turns around and says that their equity grant is now too large is just so much bullshit. Back when those shares were next to worthless and the early employees were putting in the long hours to make something from nothing, would the CEO have been so comfortable coming to those employees and telling them that they were only worth some fraction of what they had previously been offered? I highly doubt it.
Sadly, this kind of attitude has become increasingly pervasive in this industry over the last decade. The startup culture developed because employees felt a real sense of ownership of what they built, in large part because they were actually rewarded for their efforts. With the rise of companies financed almost from day one by aggressive venture capitalists, that ethic has disappeared. Now, it is all about VC's preserving as much of the value for themselves as they think they can get away with, at the expense of the folks doing the actual work.