Other thing that may be interesting to note. You can describe reversible algorithms in straight C or C++ . Instead of the assignment operator, you need to use operators ^= , += , -= which are reversible, and you would need a compiler that would be intelligent about wiping the temporary variables in the expressions by reversion of the statement. The multiplications are slightly tricky but a+=b*c can be implemented in a reversible manner.
I am a game engine programmer (with background in special effects / simulation) and it seems to me that I could easily write a neural network simulator (fairly basic) which would make use of reversible computation and would use about 4x the memory but would have a hundred million times lower power consumption for 10^14 synapses. Essentially it will only wipe as much data per step as is the input and output of conscious being, which is much smaller than the state ( assuming 1 megapixel vision and 10 000 muscles.) The exercise seems pretty trivial to me; the calculation structure would vaguely resemble a Feistel network (I drew my inspiration from it), and it would be able to run the being in reverse using some constant-size extra data that is carried along with the being, if a log of the being's original 'sensory input' is recorded.
This post may be interesting to some LWers.
In summary: it looks like our universe can support reversible computers which don't create entropy. Reversible computers can simulate irreversible computers, with pretty mild time and space blowup. So if moral value comes from computation, negentropy probably won't be such an important resource for distant future folks, and if the universe lasts a long time we may be able to simulate astronomically long-lived civilizations (easily 10^(10^25) clock cycles, using current estimates and neglecting other obstructions).
Has this been discussed before, and/or is there some reason that it doesn't work or isn't relevant? I suspect that this consideration won't matter in the long run, but it is at least interesting and seems to significantly deflate (long-run) concerns about entropy.