I think this is probably a bit extreme (I realize you may be employing hyperbole for effect but still) Zynga is a considered a "hot" IPO candidate and as such you will find folks who recognize that not everyone is a super star. Some talent might be more attracted to Zynga because they 'deal with the dead wood' in a proactive way. And pre-IPO options are better than post-IPO options.
It will certainly be interesting to see if it affects their ability to recruit and retain talent. I can only hope that some of the HR people escape and can share the results with others. Too often there isn't any really definitive data post event if it was a net positive or loss for the company. This is why I think there is a lot of hand waving in the HR space, hard to get real numbers.
Would you really classify this as "dealing with the dead wood?" If they were "dealing with the dead wood", I'd expect they would just fire them for being dead wood in the first place and get their unvested options back as a side-effect.
Trying to keep the dead wood employed, but scaring them into effectively renegotiating their contracts isn't the sort of "dealing with" that very many people are going to look upon favorably.
Yes some but I don't think much, IT employment is very much based on trends, in the 70-80's the hot place to work was Bell Labs or DEC, in the 90-00's it was Google. If you ruin your name you loose the A-Listers interest and everyone in tech fashions themselves A-Listers. So when this happens you have to rely on finding talent that has not proven itself yet or settling for the "it's just a job crowd". The biggest draw to pre-IPO is stock and the possibility of fortunes. To me and probably quite a few others, this is a shot across the bow that they will limit the participation in the fortunes part, which kills the primary reason people join pre-IPO start-ups. I would not underestimate the gravity of this action especially in this hot of a market.