Oooh, this is bad. A fellow at this year's DefCon came up with a program that exploits the weak (or absent) security of an 802.11b WiFi network. His program, airpwn, inserts HTML into an HTTP transaction. What was his favorite thing to insert? How about replacing all the images on a web page with the goatse.cx picture? (If you don't know what it's a picture of, you really, really don't want to. Seriously.)