Exactly, the technology for anonymous virtual currency already exists. But the Govt would never back such a complete anonymity. Taxation, the backbone of the govt, would be tough to enforce. Hawalas and scammers would enjoy. Now if somewhere to develop a semi-anonymous currency (like cash, with enough effort, you could probably trace it), then we probably can hope for Govt backing.
The problem with this argument is that it assumes there is but one government. Why shouldn't a small nation somewhere choose to offer financial advantages in order to lure business to its locale? (Hint: Several already do.) Why shouldn't this extend potentially to anonymous currency? If there is enough benefit to the nation (infusion of capital, prestige, whatever), then it just might happen.