So I guess the market estimates a 50% chance that they disappear with the coins. 50% arbitrage upside if you can get money in and buy bitcoins at half price, 50% loss chance if they vanish.
Most users also aren't able to withdraw fiat from Gox, so there isn't a liquidity advantage to selling your coins (which would raise the supply and lower the price). The 50% discount is probably because users believe that if Gox dies, they will have better odds of eventually recovering dollars rather than coins. These users believe the coins may have been lost, stolen, or end up unrecognized by the bankruptcy court.
Also, there is lots of speculation about price manipulation at Gox. Some users believe that Gox is artificially lowering the price to help fix their insolvency.
Before yields went to zero I had a treasury direct account and occasionally the .gov found it necessary to pay me $1000 next month (or whatever it was) in exchange for my giving them only $999.whatever today. That doesn't mean a "thousand dollar T bill" is worth less than a kilobuck, its just discounted by time. The feds discount isn't much. At least some owners of mtgoxbtc seem to think their discount is worth about three hundred bucks.
Market seems to be betting at "unlikely".
This would be a bit closer to junk bounds or penny stocks.