VR Zone reports you may need to get a new power supply if you want to use the new C6/C7 low power states of Intel's upcoming Haswell processors. These new power states require a power supply that can maintain a minimum current load of 0.05A on the CPU exclusive 12V2 rail. The previous ATX12V v2.3 design guidelines called for a minimum load of 0.5A on the CPU power rail, so a lot of older and budget power supplies will likely not meet the new specification, resulting in stability or shutdown issues once the system enters the C6/C7 power state.
Unfortunately, there's no way to know which power supplies are fully Haswell compliant as PSU manufacturers do not usually report the minimum load on their specification sheets.
Older or bargain basement power supply units may be unable to achieve this, and might become unstable or trigger a UVP/OVP shutdown once the system enters the C6/C7 power states. This is because the previous ATX12V v2.3 design guidelines for Ivy Bridge and its predecessors only called for a minimum load of 0.5 Amps on the CPU power rail, hence a less sophisticated internal feedback loop/protection could be used in older or cost-reduced offerings, which will not support the new standards.
Most worryingly, PSU manufacturers (even the enthusiast grade ones) do not usually report the minimum load on their spec sheets, so it is virtually impossible for us to come up with a list of supported units at this time. To ensure a smooth transition, mainboard designers and system integrators have been told to implement a BIOS-level toggle to enable/disable the new C6/C7 power states.