I've always liked Cartoon Network's concept of having their characters coexisting "off-camera." And you just know Zim's gotta be pissed about some of the stuff Avatar is able to get away with, like killing people off. (IGGIIIIIIINS!!)
For some reason, I get the feeling Dib slapped that "Irken Safe" label on the water cooler...
(Oh, and I'm totally going to color this when I get Photoshop back. I loved how it came out.)