Is Bitcoin a ponzi-scheme?

No, bitcoins are essentially a commodity with a fixed rate of issuance. It is not a ponzi because after you've acquired bitcoins, there is no promise extended that you can redeem them for any value and there are no dividends paid to those who hold bitcoins. Bitcoins are traded, so the exchange rate can rise or fall, so there are those who will buy or sell bitcoins to speculate on the price.

The free markets where bitcoins are traded are going through price discovery for the exchange of these. The exchange rate has fluctuated wildly over the past two years. The rate of bitcoin issuance is known in advance, so there is no chance of those holding bitcoins being exposed to devaluation from an increased supply being issued. There can be dumping at the market from those who previously bought bitcoins.

Now, there are several ponzi operators that have discovered that Bitcoin makes a fantastic payment system for their scam. The reasons include how Bitcoin transactions are non-reversible, so there's no risk of chargebacks or a payment reversal. Bitcoins do not need to be kept with a bank or any financial organization -- so the operator is not at risk of the funds being confiscated. Bitcoin transactions are made pseudonymously, so the operator's identity may be fake or remain undisclosed even. The funds are available fast -- settlement of funds occurs in about an hour, which helps the operator's cash flow.