With unity (Ubuntu 11.04 default DE), it is not possible(as far as I know, it may be possible to hack it to do it using some compiz magic), but with something else like AwesomeWM, Fluxbox/OpenBox or KDE, it is much easier. For Fluxbox, you need to add the terminal to autostart (with your command for RSS), then set the geometry and turn the window borderless. With Awesome, set the geometry, then put the window on the "desktop" level and stop managing it. For KDE, it is a simple plasmoid.
Last edited by Elv13; 01-22-2012 at 08:10 PM.