The footnote/tiny print at the bottom of xkcd.com pages reads:
We did not invent the algorithm. The algorithm consistently finds Jesus. The algorithm killed Jeeves. The algorithm is banned in China. The algorithm is from Jersey. The algorithm constantly finds Jesus.
And the following one added by Randall:
This is not the algorithm. This is close.
It was added by Randall in April 2007, according to his Blog as a response to random billboards appearing in the New York, Los Angeles and San Francisco areas. It turned out these were a viral marketing campaign by the ask(jeeves) search engine to drive publicity around their new search algorithm. The campaign is long over, but Randall keeps the text there as a self referential advertising campaign. Specifically, people who find the small text will use a search engine to see what it means and the search engine will likely lead them back to xkcd — where they saw the text initially.