Gavin, your solution would need a change in protocol though and would mean every user in the network has to update their (potentially 3rd party) client, right?
Not all, no. Just enough, which is tricky to define. If the bulk of the exchanges (which really means just Mtgox right now, but that is an accident of history and not a rule) were on the new rules, that fork would be the most useful fork. Nodes would gravitate to it, as users that wanted to cash out, or pay people that wanted to cash out, or pay people that wanted to pay people that wanted to cash out, etc, etc, would have a strong incentive to get on.
The nice thing about doing it early would be that it would make a whole class of potential 51% attacks pointless.
Perhaps that should be a little bit less far down on the TODO list after all.
Pardon my ignorance, but if the attacker has stable 51%, even if we all change our clients to support the new suggested rules, all our blocks will be still rejected, won't they?
No, because the attack blocks would be invalid, and thus not in competition for the regular blocks, at least with the nodes that have adopted the new rules.