This idea is very cool. I just got done with one over at indepth arts (http://www.critical-depth.com/cgi-bin/idd) It is a tile challenge. That means there is a theme, and everyone gets to make part of the whole image. Here's the catch: one person starts it, then the next people only get a 50 to 75 pixel wide sliver of the first tile to work off of. This way, you can tile the images so that they fit together, but you can never tell what it looks like till it's done. It's really fun.