Furthermore, given the huge amount of attention and hype about How Facebooks Success Is Assured, a lot of the "pop" in valuation that took place pre-IPO has probably already taken place. People who expect that Facebook will have its valuation (its market cap is currently estimated at $41 billion; Apple is $219 billion; IBM is $157 billion; Dell is $22 billion) "pop" by another 10x post-IPO will probably be sadly disappointed. And if you don't get that "pop", those stock options won't be worth all that much.
Which is one of the reasons I really don't believe the stories of $3.5m and $6m retention offers. It would simply be insane to make such offers, because the payout for someone jumping from Google to Facebook at this point, roughly a year before when people are guessing Facebook will IPO, are nowhere near that large (after stock reserved for the 5 stages of VC investment, and the founder's stock, and stock already issued to other employees, not to mention the shares that you know, need to actually be sold to the public, the number of options that could be granted per employee won't be that high). And, I'm pretty confident Google isn't insane. Furthermore, if Google was doing this, it would be bleedingly obvious on the quarterly 10Q statements it has to file with the SEC. So if you really think this is real, try doing some digging in the 10Q's 2-3 months from now.