A PCI-E card with two 8-pin power connectors can draw up to 375 W total. 150 W from each of the two connectors and 75 W from the motherboard connector. This will require large fans a la the latest video cards that draw this much power.
Well it can't pull any power from the motherboard. BFL claims the product can be connected by EITHER PCIe or USB. So when connected by USB that would mean it isn't connected to motherboard's powerlanes. So either it is going to be three 8 pin PCIe connectors or they will need to run it overspec. No I am not making this up.
We're in agreement. If it's plugged in to a PCI-E slot it can get 75 W from there and then draw 150 W each from two 8-pin connectors. If it's hooked up via USB it will need three 8-pin PCI-E or some other high-power connector from an external PSU because you're not getting 350 W through a USB cable :-)