The Topic says it all, although rhythmbox is part of the official gnome distribution, it's not installed by the meta ebuild.
no it isn't http://ftp.gnome.org/pub/GNOME/desktop/2.8/2.8.2/sources/
i checked here http://ftp.gnome.org/pub/GNOME/sources/ and took a look at debian's gnome meta packages. both include it, so i think it's a good idea for gentoo as well
Sorry, I shouldn't have been so short with my first response. Many projects outside of the core desktop are hosted on gnomeftp, mostly to benifit from the gnome translation team. Other distros do include rhythmbox by default, as it is a nice mature media library for gnome. Right now, gentoo-gnome doesn't provided its own idea of the desktop through the metabuild. We only include modules that have been officially accepted as part of gnome desktop platform and it is our goal to stay as close to upstream as possible.