I see some merit in what cunicula has to say. However, I don't think any central bank could abruptly gain 51% (and thus, obviously, 100%) of hashing power - it would cause an instantaneous backlash, and there would be an immediate hard blockchain fork with the bank's servers being blockaded (caveat - what if the bank connects over TOR?).  However, in the far future of bitcoin, when many people depend on the blockchain for the salaries and groceries, then a central bank that managed to gain 51% surreptitiously could quite easily begin to levy a tax. They'd have to walk a fine line between rejecting untaxed blocks mined by others, and not arousing too much suspicion. And it would be easier as bitcoin became more accepted as a mainstream currency.

I have to say, the development of ASICs has me plenty worried. Hypothesize if you will a scenario where bitcoin mining is restricted to licensed entities only. Well, everyone can still have their GPUs for gaming, and even FPGAs have legitimate uses. But ASICs that only know how to SHA256(SHA256()) could have only one plausible, and therefore illegal, use. No-one could easily compete on mining, and since building ASICs requires big factories and investment, it would be easy enough to shut down unlicensed manufacturers. Think, the Treasury would be replaced by an ASIC factory.  I would be *very* happy to see the devs change the mining protocol to be ASIC unfriendly, if such a thing is possible; even better would be to make it FPGA and GPU unfriendly, and bring back CPU mining only -- only then will mining be truly fully distributed.

Of course, if this scenario came to pass, you'd still have other miners mining away, probably on an underground black-market blockchain - like I said before: "the blackchain":