Bitcoin address
Just above the "footnote", at the bottom of the xkcd, is tiny print reading:
BTC 1FhCLQK2ZXtCUQDtG98p6fVH7S6mxAsEey
The later portion (bitcoin address) 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.