Simon, regardless of what you think is doable, you are taking unnecessarily high risks to develop such a high-TDP chip. It would have made a helluva lot more sense to go with, say, 4 chips of 63W each. The die would be 1/4th the size. 1/4th the cost too since you would have 4x more chips per wafer. A little more PCB space would be used (big freaking deal). But this would have taken away a lot of the risk of cooling a single hot chip.
The energy density (W/mm2) would still be the same. 1/4th the wattage over 1/4th the area. They had decided to go water cooling for efficient heat transfer. That would mean more complex asembly, more components, and higher cost. Not sure how that is going to improve ROI%.
There is a reason that Intel puts 4 cores on a single socket chip rather than having consumer grade boards run 4 sockets with single core processors.
Yes the energy density is the same. But my point remains: it is technically easier to cool 4 x 63W chips than 1 x 250W chip. So I am waiting for Simon's reply... If my point wasn't clear enough: an air-cooled 4 x 63W-chip Hashfast Baby Jet could almost assuredly be designed and shipped before a more complex water-cooled 1 x 250W-chip system.
PS #1: the reason Intel doesn't use 4 sockets on consumer grade boards is because sockets are expensive (4 sockets at $10 each would double the price of a consumer grade $40 mobo...) plus space is constrained in a desktop form factor (whereas nobody care really if a 400 Gh/s miner delivered in October is 1U vs 3U).
PS #2: I also disagree with BFL's choice of the PCIe form factor. I am just saying it could be worse if they had decided to host a single 300+ watt chip on the card.