Every time xkcd updates, its more idiotic fans see fit to alter the Wikipedia article in question to make sure everyone knows that there has been an xkcd comic on the topic. This blog tracks the phenomenon.
Every time a new xkcd is uploaded that references anything strongly connected to a Wikipedia article, you can bet someone will edit that article within hours. This blog tracks the Wikipedia vandalism.
Interested in helping? Alert readers can contribute by scouting out xkcd-related Wikipedia vandalism that may have escaped my view. You can also scour the xkcd archives and find older vandalism if you feel so inclined.
This isn't a negative commentary on xkcd itself (Carl Wheeler has that covered), but it is a reflection on the fanatic end of the fanbase. I'm not aware of Randall doing anything to stop it, but then again, I'm not sure he could.