For ages I have been using computer hardware that runs with -48V DC (that is: "TELCO gear",) these days I am converting all my telematic gizmos to 13.8V DC ("12V" lead acid battery float charge) bus. Instant integrated UPS within the systems.
Nothing new in having some DC bus voltage inside the systems.
The new Sun/Fujitsu M9000 mainframe system has common intermediate 48V DC bus powering all modules, and all power supplies in system feed the same bus. Conversion efficiency optimization is very important when total power consumption is up to 45 kW per machine.
In commodity-hardware the +12V DC is definitely obvious choice.
For quite a while the CPU core supply switcher has been non-isolated DC-DC converter on system board next to the CPU. Efficiency has been 94-96% in most designs, and boosting it couple percent is in the realm of possibility (ever lower output voltages at high currents is difficult enough challenge.)
Still 5% of 120 Watts is 6 Watts, which is still hefty amount to blow away with the wind, but less difficult than the 120 Watts in the CPU...
Say you get that main-to-12V DC at 90% efficiency, multiply by 95% and total system efficiency is 86% If both converters are running at 95%, the net result is 90% total conversion efficiency.
(The result is the same when there are lot of output converters.)
Is it possible to make 95% efficient mains-AC-to-12VDC supply ? Most likely it is. A 300 Watt supply would still have 15 Watt waste power to blow away.
Engineers view on the issue is that yes, single optimized converter from mains-AC-to-intermediate-DC plus a number of non-isolated very high efficiency DC-DC converters does make very much sense.
The mains supply converter has required 2000 Volt isolation (which is an added difficulty with lots of output voltages), while final converters need not be isolated at all.
Stand-by power generation can be separate 10 Watt supply in the mains converter performing at 80% efficiency without being noteworthy.
For embedded uses there is already amazingly small "picoPSU" by mini-box.com. Various models drink up 12V DC or 6-26V or whatever, and the entire power converter fits on top of motherboard power connector. The lovely wonders of "mini-itx"...