The standard academic explanation for this is that with a float lower than shares outstanding the stock is very difficult to be shorted (See Bradley and Jordan 2002) and there are no options for a period so little way for those who believe the stock is overpriced to act on this mispricing.
While I have taught that many times and truly do believe it to be correct, I am always a bit concerned when academics meet practitioners. So I was very happy to hear that they agree.
From Research 2.0 Who is eating at the OpenTable?:
"We just published a research snapshot on OpenTable (OPEN - $28.75) which includes our typical intrinsic valuation analysis as well as a view of the company. Unfortunately for current investors the company is worth about 65% less than it’s currently trading at in the market.
This is explained in good part to the fact that you can’t short the stock and there are no options as yet since they just came public. But all this does is delay the inevitable decline to much lower prices before new long-term investors can buy shares."