Among all applications currently integrated with the indicator-applet (Empathy/Pidgin, Evolution, Gwibber), Evolution is the only one without a close-to-tray functionality. This is inconsistent as it forces the user to leave it open in order to use the indicator-applet properly. Close-to-tray functionality has been implemented for Evolution before, so I guess it's possible:
http://gnome.eu.org/evo/index.php/Evolution_Tray
http://blog.sukimashita.com/?p=124