I've been thinking about small form factor computers lately for space-constrained builds. For example, controllers w/ built-in connectivity to media devices (TV, audio) or a portable console emulation device. I don't know too much about them and their flexibility when it comes to emulation.
I do know the Raspberry Pi is relatively underpowered, but will run older arcade games. But what about Intel Atom and other similar devices? I imagine they would have enough power for most types of emulation, but don't have one yet to try out...