The reason the value of bitcoins is going up is because people who buy bitcoins expect that others will pay a higher price for it in the future.
That has a much shorter name: http://en.wikipedia.org/wiki/Greater_fool_theory
The price of the Mona Lisa won't go down because the art community won't let it. It can do that because there is only 1 Mona Lisa. Such a force doesn't exist with Bitcoins.