Hence I will soon have over 25 LX150 boards and it'd be trivial to use my business-case spreadsheet to work out how much I'd be prepared to pay for a bitstream that converts my 208 MH/s units into 300-odd MH/s units.
Indeed, a hypothetical 300MH/s bitstream would make your equipment 50% more valuable for bitcoin mining purposes (let's not kid ourselves here: the non-mining market for these stripped-down boards is nearly zero). So a rational miner would be willing to pay 50% of the cost of their boards in order to get these results. Rest assured that I have no intention of charging nearly this much, although it is an important data point in assessing the value of what I'm providing.
However, unless there are miners out there with tens of thousands of LX150-based boards already running,
I think you are greatly underestimating the number of FPGAs out there that are mining. The number of people involved in FPGA mining is small, but there are already several
very large FPGA farms. Also, I have pretty good info on Xilinx' pricing curve, which lets me put a lower bound on how many units the major vendors have sold. Lastly, I've been watching AVNet's inventory data, and they are moving pretty huge quantities of LX150's compared to LX75's -- and I don't know of any other non-scientific-computing product that uses them.
I can't see any single miner paying enough to 'compensate' for the number of hours development involved,
Neither can I, which is why kickstartr is currently the most likely option.
But... all this is academic. Until there is a larger performance boost
and I have done a demo in front of independent witnesses there is neither enough data nor a real need for this discussion.
If a unit fails, and needs re-programming ... If the only way to prevent 'theft' of the bitstream would be to lock the FPGA so it can't be used for other purposes
I'm not enthusiastic about the bitstream encryption route, but I do want to point out that
this is just flat-out false. ADDING a decryption key to a Spartan in no way prevents it from being used for other purposes -- you don't have to use it (in fact, most bitstreams don't!). The encryption key is stored by writing to eFuses; Xilinx has multi-million dollar customers relying on those fuses. They are no more likely to fail than the rest of the device (in which case you're screwed anyways).
it's probable that the open-source effort will, eventually, squeeze enough hints from the thread to build their own version.
I doubt it. You really do have to start over from scratch to do an algorithmically mapped and placed design. Sure, somebody might do that, but it most certainly won't arise as a derivative of an xst-and-xilinx-map design. And even if they're twice as fast as me it will take them four months -- so if you haven't heard about somebody starting an open-source project like this today you can be sure there won't be any ready to use until at least 4Q2012.