There is also the situation that, where there is money to be "made" and the only resources you need to make it is lots of electricity and some high power computers then the "big boys" will step in. We will have companies (or other associated financial groupings) who can afford the initial outlay on the equipment and as result will reap bigger rewards than your average Joe with a spare PC. This will create a "class system" where the rich will simply get richer, and in doing so will have a currency that can't be easily tracked allowing for all sorts of untraceable financial misdemeanour's. Now while not every company wants to avoid tax, or has a need to syphon off money to nefarious operations, but the truth of the matter is the crash has shown that a large percentage of them when given the chance, have.
I don't think virtual currencies are not possible, in fact I think they are an integral way in resolving a lot of the problems that have developed in the modern global economy, but Bitcoins as they are (and imo) I have some difficulty in believing they can succeed and more so if they do they don't open the door to the issues I mentioned.