I agree with methusaleh. It's not hard to change the hashing algorithm by a power factor necessary to make sure that even quantum computers would require millions of years to crack encryption. Mining adjustments would have to be overhauled as soon as it was recognized that quantum computing could crack encryption and there might be network stoppage until everyone upgrades to quantum computing.

As for consensus on the bitcoin protocol, I'm inclined to think that bitcoin is the Yahoo! of finance and might no longer be dominant by the time quantum computers become reality. But if it's around and everyone realizes they will lose their wallets without an upgrade, I believe they will be very fast about it. By no means does it mean the end of cryptocurrency, but it will cause its evolution.