Bitcoin address
Just above the "footnote", at the bottom of the xkcd, is tiny print reading:
BTC 14FHqYSgAi39CEJksUJJsK8JzJzyqFpLVk
The later portion also appears at http://xkcd.com/bitcoin/. "BTC" is the abbreviation for bitcoin. This is apparently xkcd's bitcoin address, perhaps the high-tech equivalent of a tip-jar.