I have a lot of Raspberry Pis... they all use 5V PSUs. A few months back I bought some Chinese wall worts and used them with my Pis. The Pis ran fine... but when I happened to touch ground on one of the Pis and another grounded metal object nearby I got a massive continuous shock. A multimeter showed that 240vac was present between the USB charger's ground and earth. So now I make sure I test wall worts before use.

So yes, you could put 500v down a standard USB cable, the voltage isn't the issue, it's the Amps. A prolonged short circuit at high voltage will draw enough amps to destroy the cable.

I ordered a 30GH unit from BFL... I didn't think they'd deliver and it took ages... but to my astonishment they did deliver and the machine in question is working just fine. The PSU of the machine is big, more than adequately specc'ed for the purpose and certainly isn't "leaking" electricity.

My only gripe, apart from the delays, is that the unit I ordered and received didn't have a strong enough airflow to keep the chips running cool enough to avoid thermal throttling (at 21 deg C), but removal of the grilles on either side of the case fixed that problem.

BFL has made mistakes... Blackmagic Design has recently too with their incredibly late 4k cameras... but I think a lot of the issues BFL has been experiencing are self-fulfilling prophecies (our self-fulfilling prophecies).