Windows has tooltip balloons, MacOS X has Growl. What do we have?
MikeHearn and ChristianHammond kibitzed on this last summer, and produced:
There is also a protocol specification in DocBook form. However it makes no sense to try and get this into GNOME while DBUS is unstable. Also the code needs a bit more work, and an X composite based eyecandy frontend would not go amiss either.