of course pluto needs an OS, we use debian sarge.
yes, you can use the admin website to choose all the attributes for a media file, including cover art.
you can use a bluetooth mobile phone (symbian series 60 or windows mobile 2003) or an ir remote to control pluto.
you don't need any downloadable plug-ins, maybe an update from time to time.
for a list of features you can read our commercial website, for technical details see the support website.