I would guess that he thinks that the probability of this hypothetical---worlds in which brain scanning isn't possible---is pretty low (based on having discussed it briefly with him). I'm sure everyone around here thinks it is possible as well, it's just a question of how likely it is. It may be worth fleshing out the perspective even if it is relatively improbable.
In particular, the probability that you can't get a functional human out of a brain scan seems extremely low (indeed, basically 0 if you interpret "brain scan" liberally), and this is the part that's relevant to most futurism.
Whether there can be important aspects of your identity or continuity of experience that are locked up in uncopyable quantum state is more up for grabs, and I would be much more hesitant to bet against that at 100:1 odds. Again, I would guess that Scott takes a similar view.
Hi Paul. I completely agree that I see no reason why you couldn't "get a functional human out of a brain scan" --- though even there, I probably wouldn't convert my failure to see such a reason into a bet at more than 100:1 odds that there's no such reason. (Building a scalable quantum computer feels one or two orders of magnitude easier to me, and I "merely" staked $100,000 on that being possible --- not my life or everything I own! :-) )
Now, regarding "whether there can be important aspects of your identity or continuity of exp...
Scott Aaronson has a new 85 page essay up, titled "The Ghost in the Quantum Turing Machine". (Abstract here.) In Section 2.11 (Singulatarianism) he explicitly mentions Eliezer as an influence. But that's just a starting point, and he then moves in a direction that's very far from any kind of LW consensus. Among other things, he suggests that a crucial qualitative difference between a person and a digital upload is that the laws of physics prohibit making perfect copies of a person. Personally, I find the arguments completely unconvincing, but Aaronson is always thought-provoking and fun to read, and this is a good excuse to read about things like (I quote the abstract) "the No-Cloning Theorem, the measurement problem, decoherence, chaos, the arrow of time, the holographic principle, Newcomb's paradox, Boltzmann brains, algorithmic information theory, and the Common Prior Assumption". This is not just a shopping list of buzzwords, these are all important components of the author's main argument. It unfortunately still seems weak to me, but the time spent reading it is not wasted at all.