My reaction to Ben Laurie's papers:
Incentives matter.
Yes, it is true that we cannot be absolutely, positively safe against a 51% attack unless we are absolutely certain 50% or more of the world's entire computing resources are dedicated to Bitcoin mining.
So what?
The natural incentive for somebody with lots of hashing power is to profit by playing by the rules, NOT to cheat.
And if you assume that your attacker is Rich and Powerful but Economically Irrational, then any alternative system that you propose will almost certainly be at least as vulnerable as Bitcoin. Create a system that requires 500 semi-trusted "mintettes" that all agree on a transaction log and then imagine 251 Special Agents infiltrating and corrupting the organizations that run those mintettes.
Increase the number to 40,000 mintettes to make it harder... and you've just re-invented Bitcoin.