I'd be up for building my own Ouya, but only if the software/OS was open source so that I could get whatever version of an Android store they're going to provide (wouldn't want to be on a platform that didn't have games built specifically for it), and there was a compatible gamepad that worked out of the box. Anything less and it'll be a better use of time/money to just buy the Ouya outright, assuming they deliver on releasing it to market.
I am working on an emulation box based on RPi. Many people have successfully got the software running on it, but they all seem to be "build it yourself" setups without a lot of instructions or a distribution model. I'd really like to build a stock image with the emulators pre-installed (no ROMs though, of course), that provided a "turn on and play" experience. I'm fairly close to that already, but need to work out some performance problems. Already found a supplier of replica NES controllers - yes, the ergonomic nightmare models.