I firmly believe that Bitcoin's eventual success will also be it's downfall. Once it becomes so popular that the US Government perceives it to be a threat (no taxation, no control over monetary policy, and it's potential for illegal use & money laundering) they will simply outlaw it's use in the US. If you think they won't, you should take a closer look at their draconian and painstaking efforts to control the financial system. By this I mean forcing US banks to report cash transactions, taxing US citizens income abroad, efforts to shutdown offshore money havens, pressuring foreign governments to comply with their rules in the name of controlling terrorism, and more. This isn't crazy conspiracy talk, this is fact. When it comes to money, the US government is, frankly, the neighborhood bully.
Yes, there will still be ways to get around the ban. We'll all be able to keep using Bitcoins. But it will stop Bitcoin from going mainstream and being accepted by mainstream internet merchants -- which is exactly what is needed to make it function as a viable alternative to inflation-sensitive centralized currencies.
I fear it will always be just for us geeks