Sub-prime real estate was a scam. Huge companies with massive legal departments protected themselves from the illegal part of the scam - all those loan officers telling people to lie on their loan applications. In fact these banks got their lobbyists to get them TARP bailouts after the scam had run its course.
Banks are willing to trade subprime "liar loans" once legally insulated, so big companies willing to deal with Bitcoin doesn't really mean much.
I didn't say it was impossible to short Bitcoins. They are not FX traded alongside the dollar, euro, yen etc. though.
The central question is, why do Bitcoins have value? Why did subprime real estate loans, pets.com stock and Dutch tulip blubs have value? Actually, both these things were hypothetically more valuable than Bitcoins. Bitcoins are worthless.
When the price of a Bitcoin drops below $100, are people here going to learn a lesson about how value works? I don't think they will. That's the thing - Bitcoin will crash, and those talking it up here won't have learned a thing. If they did, these scams (subprime, pets.com stock, dutch Tulip Bulbs) couldn't keep popping up over time suckering people in. People have no concept of the relation of price to value, and thus are easily fooled.