As reported here. From the article:

Rodriguez's name appears on a list of 104 players who tested positive for performance-enhancing drugs in Major League Baseball's '03 survey testing, SI's sources say. As part of a joint agreement with the MLB Players Association, the testing was conducted to determine if it was necessary to impose mandatory random drug testing across the major leagues in 2004. When approached by an SI reporter on Thursday at a gym in Miami, Rodriguez declined to discuss his 2003 test results. "You'll have to talk to the union," said Rodriguez, the Yankees' third baseman since his trade to New York in February 2004. When asked if there was an explanation for his positive test, he said, "I'm not saying anything."

Personally, I don't particularly think this is a big deal; I've suspected for a while that he might have been a user because like many other players in this era, he blew up. But considering he's pretty much the player on pace to break the all-time HR record, this isn't going to wear well on him unless he manages to do what pretty much every other player weighted with this accusation has failed to do; explain it away.