The interesting issue with reversible computation is that you do not need complete reversibility to reap nearly all the energy saving benefits of reversible computation. For example, a CPU built on perfect reversible logic, which would run normal irreversibly computing software, would still have ridiculously low energy requirements compared to today's CPUs, as most of the irreversibility happens on the very low level (think intermediate steps when adding two numbers, think loss of state of each transistor in CPU at every clock cycle) and at the level of software the number of erased bits per cycle is actually rather small. edit: and can probably be further minimized by optimizing compiler without changing behaviour of program in any way or changing it's memory requirements by more than a constant factor
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.