The version of NowPlaying in the package of Screenlets from Ubuntu jaunty 9.04: 0.1.2-ubuntu2 doesn’t work with Rhythmbox.
Julien Lavergne provided a fix which isn’t yet available in the Ubuntu main repository but you can download a fixed version of the package from his PPA: 0.1.2-6~ppa6. Checkout this doc from launchpad to add a PPA and its key to your Ubuntu repositories.
BTW: NowPlayingScreenlet theme pack ship some really nice skins – Vinyl especially – to be untar in: /usr/share/screenlets/NowPlaying/themes
