The FPGA stuff sounds like a really interesting investment and in the (unlikely?) event that the bitcoin stuff doesn't pan out then I'm sure you could sell the rig to an internet security company or something. 3 months to make your investment back really doesn't sound bad, though.
You can also get FPGAs mounted on boards with all kinds of interface options, USB and whatnot, ready-made from the manufacturers and those are really easy to program if you want to do it yourself. They're more expensive, though, since you're paying for all the extra hardware as well as the FPGA, but updating and sharing the algorithm would be a walk in the park. Should be great for people who already own an FPGA board, or people who want to get one anyway for the reprogrammability.