MLB today released the following statement.
The Office of the Commissioner of Baseball announced today that San Francisco Giants outfielder Melky Cabrera has received a 50-game suspension without pay after testing positive for Testosterone, a performance-enhancing substance in violation of Major League Baseball’s Joint Drug Prevention and Treatment Program.The suspension of Cabrera is effective immediately.
Melky Cabrera who was 1st in the NL in hits (159), Runs (84) and 2nd in Batting Average (.346) dealt a serious blow to the Giants offense as they push to make the post season. After the news broke Cabrera through the Players Union and the Giants  on their site released the following statements.
We were extremely disappointed to learn of the suspension of Melky Cabrera for violating Major League Baseball's Joint Drug Prevention & Treatment Program. We fully support Major League Baseball's policy and its efforts to eliminate performance enhancing drugs from our game.  Per the protocol outline by Major League Baseball's collective bargaining agreement, the Giants will not comment further on this matter. - SF Giants Press Release
 
My positive test was the result of my use of a substance I should not have used. I accept my suspension under the Joint Drug Program and I will try to move on with my life. I am deeply sorry for my mistake and I apologize to my teammates, to the San Francisco Giants organization and to the fans for letting them down. - Melky Cabrera
While Cabrera accepted his suspension it should be noted that it came after he exhausted his appeal process. In fact back on July 27 Andrew Baggarly who covers the Giants for CSNBayArea.com tweeted about a rumor that Melky Cabrera would be suspended for a positive PED test and that he asked Cabrera directly and Cabrera denied it.Cabrera not only denied it but he went on to suggest that perhaps a Dodgers fan created the rumor as a way to distract the club as they were heading into the big, three-game series between the two rivals while crediting his success to hard work. This along with his statement after he was named the 2012 All Star Game MVP when asked if true what made him recommit himself to baseball where his reply was
I think the one person that has the most influence on me is the Lord. He is the one that embraced me in terms of playing better, my family, my mom, my grandmother, my girlfriend, and my attorney that spoke with me and told me that I needed to concentrate 100% in baseball, and also my personal trainer with whom I work during the off season.
we now know it was about a commitment to Testosterone.According to MLB Cabrera can still play this season if the Giants make it deep enough into the playoffs as the Giants have 45 games remaining on the season and they would need to play at least 5 games in the postseason before Cabrera could be eligible.