We start with an assumption that there exist a financial market which allows one to profit when a price of a certain currency drops.
Recall that short selling is tightly regulated, for that exact reason. If such a market did exist, it'd probably be a black market (for now, at any rate). Anyway, you could apply the same argument to Bitcoin itself. The network is not that hard to DoS, so, perhaps everyone should just give up
It's easy to over think things. When Bitcoin was new a lot of very smart people I talked to about it came up with very reasonable and convincing arguments about why it couldn't work. All those people were wrong. By all means think things through, but be careful of falling into the trap of being too clever!
Don't CampBX and Mpex support short selling already?
OTOH, short selling requires credit, thus trust, something hard to come by in black markets.
