For large, conservative investors, this is the first chance to buy bitcoins directly from the U.S. Government.  There is an implicit guarantee that the coins to be sold are legal and without any sort of taint, at least in the eyes of the Feds.  Also, to many such investors, the U.S. Gvt is the most trusted trading partner possible.  They are the most reliable organization to trust wiring millions of dollars to.

The auction is a giant stamp of approval, if not for bitcoin as a whole, than at least for the particular set of coins at auction.

There are obviously reasons why the coins may sell at a discount as well, and I tend to think that is more likely.  My guess is there is about a 20% chance the coins will sell over spot.  After all, the supply of government-approved bitcoins is very very tiny.