That's Kryptos [wiki], a sculpture by James Sanborn on the grounds of the CIA headquarter in Langley, Virginia. The sculpture is a code, which is unsolved for over 27 years.

Kryptos has four encrypted passages, three of which had been cracked (in 1998, CIA cryptoanalyst David Stein cracked the code using only paper and pencil!)

Supposedly, Kryptos is the subject of Dan Brown's (of The da Vinci Code fame) next book.

If you like this kind of stuff, head on over to The World's Ten Most Famous Uncracked Codes - Thanks wade7!