Really, it can be about anything. I suggested using this strip
as a base guideline, and people took off and did their own crazy (but good) thing.
Most people so far have included themselves as characters in the guest strip, and subjects range from locking their front door, voting in the local election, having men turned into lust-zombies for Johnny Depp, to making a mess while inking (Kazu's comic, which will run this Monday).
Eeenh, it can be about anything! I'm not at all picky -- Just have fun!