Just from reading this thread, I suspect what the Bitcoin folks are suggesting is to make custom processing hardware (in the FPGA), and simply use the R-Pi as a frontend to it. I see nothing wrong with this at all - the GPIOs are there for just this sort of reason. Quite likely an R-Pi plus FPGA kit is better value for money (and per watt) than a full PC running a GPU, for this specific application.
I know nothing about Namecoin, but the R-Pi should be fast enough to perform many light server duties, and it also has the advantage of being light enough on power to make a good ghetto UPS for (burglar-alarm battery, car battery charger, in-car phone charger, wire them all together, done). I use an old PowerBook G3 of roughly equivalent CPU power (lower clock speed but better IPC) as a main firewall, complete with DNS and Web caches, and it keeps up quite well with an ADSL2+ line.
The R-Pi would probably do less well as a fileserver, because all of it's data has to go through a single USB link. But again, for very light duty it would still work, so you could keep your music collection attached to it.
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