For reference, Antec Phantom 500 repeatably and reproducibly shuts off when feeding two 8 series NVidia cards working in SLI.
Conclusion? Need to pay more attention to the general acoustic qualities of the case and its ability to suppress the wind noise. Slight hope for improved energy efficiency of future components, but in any case I don't think there'll be fanless PSUs able to feed high end gaming systems anytime soon.
It seems, by the way, that Antec Phantom 500 was the most powerful fanless power supply ever manufactured. Whereas it does have a very annoying auxiliary fan (very noisy) which switches on under heavy load (which gaming definitely is), using it for HTPC is perfectly justified - I don't remember the fan ever switching on.
It is still being sold online, so hurry before existing supply runs out...