How can you play this as an investment? You can't! Unless you are a very large (or important) high net worth individual (HNWI) at Goldman Sachs ex-US according to an earlier Reuters report. It was rumored that Goldman offered HNWIs the chance to invest a minimum of $2M with a lockup until 2013. Goldman was estimated to raise $1.5B in the private placement.
Although it has come to light there is an alternative market trading pre-IPO private shares. Sharepost is reportedly trading private shares of Facebook, Twitter, and Zynga to name a few. In order to qualify, the website states that you must be an accredited investor. The current Sharepost Facebook implied valuation is $73B. Without data it's difficult to determine the proper valuation. However if the investor were able to get in at $73B and it IPO'd at $100B, the gains will be sizable.
For all the remaining retail investors, you will have to wait until the IPO hits the secondary market.