There's two options I think: 1. a machine for the central hackage server, 2. a machine for doing package builds
The former will require more organisation, partly because we need the haskell.org people to have some degree of control over the system. The latter is easier because the design allows for multiple clients to do builds rather than just one central machine. So all that requires is a user account to upload the data. (plus the small matter of a working build bot client software, which is where scoutess may help)
I wonder if this could get to the point where it could be done seti-at-home style, farmed out via a VM image. That is people would run the image to provide resources (and geographic distribution) to the build server cloud. Maybe they get a fast local mirror as a reward. If it were every that easy I would certainly love to run a VM! -Ryan