Depends on the way they'll do this, one way is that they could approach some major stakeholders of Facebook (e.g., a pool of employees with vested or unvested employee stock options) and draw up some agreement with that entity to buy the rights to buy these stock options at a whole-sale strike price (let's say $0.05/share to buy the share at $75).Then the brokerage firms will chop these options to different strike price bins and re-distribute to investors (e.g., $0.15/share for the right to buy the share at $75, $10.15/share for the right to buy the share at $65; with the broker collecting a $0.10/share underwriting fees.).
So you are buying an option at a certain strike price to buy a potentially unvested option to buy a illiquid, pre-IPO stock at a certain strike price. If this was Google in 1999, someone is going to be rich. But since it is my opinion that Facebook is going to the way of AOL, I wouldn't buy into it; but then again, it depends on when Facebook is going to IPO and if it is before the Web 2.0 exuberance dies down.