I wish Thinderbird had a tray feature.
 

So it would get off my taskbar and sit nicely in the background like GAIM does. Does anyone know if there are plans to do this?

im not sure, i dont use the program.
but the reason gaim is able to be sent to 'system tray' is because you have a plugin installed for it. gaim doesnt do it on its own.
maybe search for a plugin like this for the program youd like?