I'm jumping on BoingBoing's bandwagon here and linking to this story in the The Kansas City Star about the "Toynbee tiles." The tiles are these bizarre little things that say "Toynbee idea KubricK's 2001 Resurrect Dead on Planet Jupiter." There are variations including one that says "Murder all Journalists I beg you" (a note of particular to concern to me.) There's a Web site at www.toynbee.net that keeps track of tile sitings. They list tiles in Philadelphia, Pittsburgh, Kansas City, New York City, Chicago, Boston, Baltimore, Aberdeen, Edgewood, Washington D.C., Columbus, Toledo, Cleveland, Atlantic City, St. Louis and Detroit. Also, there was one each in Brazil, Chile, and Argentina.
The tiles are some hard substance that is actually embedded into the pavement. According to the Kansas City Star, it would take some heavy duty equipment to meet that need.
This obviously could be somebody pulling a weird prank to attract attention. But no one has taken responsibility for it. Also, the tiles have been appearing since the late '80s.
There must be some explanation behind it, but I'm not sure I want it answered. The mystery is so compelling.