I came across an interesting news story posted by The Tech Report this morning that dives into the possibility of problems with Intel's upcoming Haswell processors and currently available power supplies. Apparently, the new C6 and C7 idle power states that give the new Haswell architecture benefits for low power scenarios place a requirement of receiving a 0.05 amps load on the 12V2 rail. (That's just 50 milliamps!) Without that capability, the system can exhibit unstable behavior and a quick look at the power supply selector on Intel's own website is only listing a couple dozen that support the feature.
This table from VR-Zone, the source of the information initially, shows the difference between the requirements for 3rd (Ivy Bridge) and 4th generation (Haswell) processors. The shift is an order of magnitude and is quite a dramatic change for PSU vendors. Users of Corsair power supplies will be glad to know that among those listed with support on the Intel website linked above were mostly Corsair units!
A potential side effect of this problem might be that motherboard vendors simply disable those sleep states by default. I don't imagine that will be a problem for PC builders anyway since most desktop users aren't really worried about the extremely small differences in power consumption they offer. For mobile users and upcoming Haswell notebook designs the increase in battery life is crucial though and Intel has surely been monitoring those power supplies closely.
I asked our in-house power supply guru, Lee Garbutt, who is responsible for all of the awesome power supply reviews on pcper.com, what he thought about this issue. He thinks the reason more power supplies don't support it already is for power efficiency concerns:
Most all PSUs have traditionally required "some load" on the various outputs to attain good voltage regulation and/or not shut down. Not very many PSUs are designed yet to operate with no load, especially on the critical +12V output. One of the reasons for this is efficiency. Its harder to design a PSU to operate correctly with a very low load AND to deliver high efficiency. It would be easy just to add some bleed resistance across the DC outputs to always have a minimal load to keep voltage regulation under control but then that lowers efficiency.