I too have been wondering about this. My thought was to have an Android tablet be another "zone" of sorts. Wifi networkable. But then, it simply needs a capable media player and navigate the media folders in a pretty way.
I know you mentioned simply as an input device, and maybe that's because, like I have found, I cannot find a media player for Android that begins playback before the whole file has been downloaded (except maybe flash, which means transcoding server on the main box AFAIK).
Would certainly make for simple multi-zone installs if there were an Open Mobile front end for Android tablets!!