I sent ShadowOfHarbringer's some of my thoughts on this in this private message:
@s{quotedtext}
@s{quotedtext}
I don't worry much right now about economically irrational, "I'm going to spend millions of dollars to disrupt the bitcoin network" attacks because I don't think anybody is going to spend millions of dollars to disrupt our tiny payment network.
I have no idea what bitcoin payments will look like in 5-10 years; I expect all sorts of trust mechanisms and relationships to develop that are independent of the bitcoin network, and I suspect some of those will make 51% attacks irrelevant.
And I have no idea what the mining landscape will look like in 5-10 years; if thousands of companies around the world are installing bitcoin mining hardware for free in every house built in cold climates (generate bitcoins and get a discount on your heating bill) then it may be completely inconceivable for even a government to amass enough hashing power to mount a 51% attack.
So while I encourage y'all to keep thinking about it as an interesting theoretical problem, it is only slightly higher on my personal priority list than worrying about quantum computers breaking ECDSA.