HOUSTON, TX, Sept. 22, 2009 - The St. Louis Cardinals announced today that they have terminated the contract of outfielder Wagner Mateo with whom they agreed to terms on July 2.

Mateo's contract was contingent on a physical exam. Results of the physical exam discovered pre-existing injuries and physical defects that led to today's decision by the club.
http://www.stltoday.com/forums/viewtopic.php?t=662322

From Bernies Twitter

The kid Wagner Mateo apparently has a degenerative condition that will greatly impact his vision...pretty serious.
That really sucks for Mateo and his family. He had a winning lottery ticket just ripped out of his hands.