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!
http://www.wired.com/science/discoveries/news/2006/04/70701
And really, Lemons, all language is a code, isn't it? A way of encoding thoughts into speech? ;)
I thought I would have at least 40 years before someone called me ancient.
http://shugborough.info/chapters.php?id=1