This may be pretty obvious to some fans of Jhonen's comic books, however, I think its just as cool as the Z? sighting or any of the sightings of the shows creators.

In the beginning of the episode 'Dark Harvest', he is seen floating in a jar. The teacher picks up the jar and puts it by the window, where a dodge ball crashes through and breaks it, as well as decapitates Filler Bunny. This is keeping in the spirit of the comics, where Filler is constantly killed, mutilated, etc etc.

Also, he makes a small cameo appearance on the dashboard of the bus Zim and Gir ride on in Walk of Doom.