The very definition of a Ponzi scheme. This will only end in tears.
-Matt
While I agree with much of your post, Bitcoin is not a Ponzi scheme, and certainly not it's 'very definition'. "Ponzi scheme" seems to be one of the most misused expressions in economics.
A Ponzi scheme is a specific type of fraud where a party pays people back their own capital rather than making legitimate returns. By definition, it is unsustainable. That's why it's a fraud.
Bitcoin, on the other hand, is just a commodity/currency in a bubble. Period. There are no mathematical reasons that it couldn't be perceived to be at its fair value of $166 forever, so long as people perceive that value to be correct.
Just because unscrupulous speculators make money off the naive, doesn't make it a Ponzi scheme. It just makes it yet another average financial marketplace, which have been drawing in suckers since 4000 B.C.