Actually there's a lot of doubt. Bitcoins are not a pyramid scheme, and whilst they could be used as a method to circumvent tax, any skilled accountant can already suggest a dozen other means to achieve the same thing.

Bitcoins are probably most at risk from being classified as a type of security, which would put them under the scope of a large number of financial regulations. It wouldn't make them illegal, but it would make them more difficult to trade.

However, in general I'm optimistic about Bitcoin's chances. I certainly don't believe it is inevitable that they will be regulated out of existence.