See here.. This guy has poked into a bunch of devices. Many run Ubuntu, which seems like a reasonable choice, but OpenWRT is pretty surprising. Of the multitude of operating systems designed precisely for embedded systems, they choose a router OS.
Not only a router OS any more. Compared to Router OS, for example.
openwrt has the big advantage, that you also can include lot (most ?) of regular, well known Linux packages. Or do you own code, to be included. And, still it is small, opposed to Ubuntu.