“With AllTray you can dock any application with no native tray icon (like Evolution, Thunderbird, Terminals) into the system tray. A high-light feature is that a click on the “close” button will minimize back to system tray. It works well with Gnome, KDE, XFCE 4*, Fluxbox* and WindowMaker*”
I love it, and the fact that is has been added to Feisty is great.
Its available in the universe repository of Ubuntu Feisty:
sudo apt-get install alltray