Forgive me if I'm wrong, but in a world where quantum computing becomes actually prevalent, as opposed to just plainly possible in a laboratory setting, there is nothing to stop Bitcoin from leveraging QC for it's encryption. This would kind of ruins the argument of ever more sophisticated QC cracking current cryptography; the new cryptographic possibilities that QC could achieve will eventually be available to the Bitcoin dev team, as well as the general public.
I'm not so sure about that. In Bitcoin you need a public key cryptosystem. In elliptic curve cryptography, the trick is that the direct problem (computing the public key or the signature) is easy if you know the private key. However, if you have the public key there is no known efficient method to deduce the private key (with a classical computer).
If everyone had quantum computers, both problems would become easy. You cannot simply scale the numbers as you would with hashing. You need asymmetry.
There are a few alternatives under study:
http://en.wikipedia.org/wiki/Post-quantum_cryptographyThe important point is that, as opposed to symmetric systems, where you can just scale everything, you need some asymmetric problem. With quantum computers there are less available problems like that.