What's the difference between a financial bubble and a Ponzi scheme ?

None, really. They both rely on greater fools to inject ever greater amounts of money into the scheme for it to work and make EVERYBODY rich.
Here's the major difference and problem: Ponzi schemes are of course illegal. Financial bubbles on the other hand ?
They were tolerated by the Fed (remember chairman Greenspan stating that he is not in the business of pricking bubbles ?) and now that the biggest one of them all has burst, the Fed is frantically promoting bubbles so that the game can go on !
In a sane economy that uses money of real value, rising prices cause demand to decrease and production to increase. In our bubble economy, when prices rise, everybody tries to buy before prices go up even higher.