The problem with 48V, so I've read, is that it is not as efficient stepping down to the lower voltages (3.3V, 5V) that are still heavily used as 12V. So, you end up losing some of that PSU efficiency once you get to the MB. According to Google and others, having just 12V out of the PSU would be the most efficient without redesign of the overall system.
But converting 240V (or 120) to 48V is more efficient than going down to 12V, plus the reduced losses in the wiring will make some of that back. But the main reason I suggested it is that the price of copper is only going to rise in the future and will eventually begin to outweigh any difference in efficiency.
Come on guys, don't you recognize a throwaway let-smake-some-noise-for-traffic coment when you see it? Talk is cheap. 95% efficiency is not. To take PSU efficiency from the mid-80s where the best ATX ones are now to >90% will take a great deal of $$$. Expensive parts, even parts that even aren't made by component makers yet because there's no demand -- except in weird military gear where $ is no object.
We're not going to see >90% efficiency any time soon at anything like a relatively reasonable price.
95% is very high, but I can see it being possible in the next few years. The specs of the components that go into these things (MOSFETs, diodes and capacitors particularly) are improving all the time, and the controlling electronics are becoming more intelligent. A lot of improvement can come from more careful design with existing components too.
Reasonably priced 90% efficient PSUs already exist at the industrial level, just not in ATX PSUs yet. I know the last few percent are the hardest, but even 90% would have seemed incredibly high not long ago.