http://www.nytimes.com/2008/05/26/business/media/26link.html
“People are generally surprised that we make a living from it,” Mr. Munroe said. Without being specific, he said that the sales of xkcd merchandise support the two of them “reasonably well.” He said they sell thousands of T-shirts a month, either of panels from his strip or in their style, as well as posters.
http://query.nytimes.com/gst/fullpage.html?res=9B0DE0DF1738F...
(sorry for the long link)
Reads like many of the "publishing will die" stories circulating today. States back then that merchandising accounted for 50% of revenue.
If you can make something novel, ala AirBNB's Obama O's cereal it seems like an easier path to ramen than AdWords.
Merchandising seems questionable, though. Consider: You have a business, and you're going to sell merchandise to promote your business. Now you have two businesses.
This strategy probably doesn't make sense for a funded startup, however.