If speculators decide they've had enough and start selling, then the bitcoin will return to a value based on its usefulness for transactions, instead of its attractiveness as an investment. How far it can go down depends on the size of consumer bitcoin demand vs investor bitcoin demand.

http://bitcoincharts.com/charts/mtgoxUSD#tgSzm1g10zm2g25zv

This is the value over a larger time. Sustainable?