I have recently been in a mood to make something with someone. There is something particularly satisfying about working closely with another artist to make something grand in concept, although it doesn't necessarily have to be something grand in scale. If anyone else is feeling this urge, drop me a line.
I've done swapping off in stages ( i.e. someone does sketch/lines other person colors set up) but I'm also up for those time trial sort of deals as well (i.e. you get two hours to work on the honor's system and then you have to pass it off to the other person wherever it ended up when your time finished).