I would find it unlikely that ghash would ever actually control 51% of the mining capacity of the network. The last time they approached 51% the price of their GH/s rose substantially and their share of the network declined. This would lead me to believe that they repurchased some of their traded capacity and took it offline.

I don't think the price of BTC would be affected if ghash got 51% of the network as long as there is not an attacked against the network. Having 51% of the network would only allow them to possibly launch an attack, it does not mean they will launch an attack.

ghash (and cex) have a lot of money invested in the bitcoin economy and would stand to lose considerably if faith in bitcoin were to be reduced substantially