Thanks for the explanation, it seems like I'm not wildly misreading wikipedia. :)

It seems like the more qubits are required for this attack, the more likely we are to have a long warning time to prepare ourselves. The other attack of just cracking the pubkey when a transaction comes through and trying to beat the transaction, seems vastly more likely to be an actual problem.

Do you have any idea how I'd go about estimating the number of qubits required to implement just the SHA256(SHA256(...)) steps required by mining?