It doesn't meet the traditional definition, no, which is why I referred to it as a "distributed Ponzi scheme" - it's closer to an MLM in classical parlance.
[edit] Here's the SEC definition:
> A Ponzi scheme is an investment fraud that pays existing investors with funds collected from new investors. Ponzi scheme organizers often promise to invest your money and generate high returns with little or no risk. But in many Ponzi schemes, the fraudsters do not invest the money. Instead, they use it to pay those who invested earlier and may keep some for themselves.
> With little or no legitimate earnings, Ponzi schemes require a constant flow of new money to survive. When it becomes hard to recruit new investors, or when large numbers of existing investors cash out, these schemes tend to collapse.
And here are the red flags that the SEC warns folks to watch for:
> High returns with little or no risk.
> Unregistered investments.
> Unlicensed sellers.
> Secretive, complex strategies.
[edit2] So with that in mind, it meets many (but not all) of the classic definition elements.
1. All returns are generated by bringing new money into the system. The only way someone can make money on Bitcoin is if a new person invests. That money is then distributed to the earlier participant, net of miner fees.
2. There is no actual business underlying.
3. The constant flow of new money is required to keep the price from collapsing because miners extract something like $50M in welfare from the system per day.
4. The entire space is unregistered investments hawked by unlicensed sellers.