We have 1 Pi at home running RaspBMC and it's great. The thing I like most about the Raspberry Pi is that you can swap out SD cards and reboot, and suddenly have a totally different machine, doing something totally different.
That's something I was wondering about: is there some state inside the Rpi itself? If not, then a system backup/restore is only a matter of dding on the SDcard.
Plain Raspbian for the NAS/music player, plus Samba, mpd, minidlna.
I am not perfectly satisfied with RaspBMC however. I'd rather build the thing myself, piece by piece, so I have only what I need and know what to change when my needs change.