Thanks for the detailed and helpful post - the sort that makes ElReg worthwhile. If I may be permitted a quibble, it's a slight exaggeration to claim that "working quantum computation blows away much of the cryptographic basis for e-commerce".
Asymmetric ('public key') cryptography is mainly used for exchanging the secret keys - 99% of Internet cryptography is carried out using symmetric ('private key') methods, such as AES, and their effectiveness isn't affected by quantum computing. For the asymmetric part, there are numerous alternatives that don't use factorisation as their 'trapdoor' function (easy to do one way, hard to do the other), and at least some of these should be immune from exploitation using quantum computers.
So if there were working quantum computers powerful enough to break standard asymmetric crypto, we'd just need to implement a new algorithm for key exchange; There would be a problem if someone had captured lots of old, encrypted traffic (I wonder who might do that), as they would be able to decrypt it easily (rather than, as at present, with great difficulty).