A statement from the journal's Editor-in-Chief Bruce Alberts detailing the retraction says:
"...Science has lost confidence in the Report and the validity of its conclusions. We note that the majority of the authors have agreed in principle to retract the Report but they have been unable to agree on the wording of their statement. It is Science’s opinion that a retraction signed by all the authors is unlikely to be forthcoming. We are therefore editorially retracting the Report. We regret the time and resources that the scientific community has devoted to unsuccessful attempts to replicate these results."
Bloomberg nicely summarizes the backstory here:
In December 2010, the American Red Cross stopped accepting blood from people diagnosed with chronic fatigue because of the risk it carried XMRV. The syndrome is characterized by severe, continued tiredness not relieved by rest. In October, scientific reports from the American Association of Blood Banks in San Diego indicated that XMRV isn’t transmitted through blood.
At least 10 trials since the study have failed to replicate the results, and subsequent research has indicated the blood samples used in 2009 were likely contaminated by the virus in the laboratory. While the majority of authors agreed to the retraction, they have been unable to word a statement on their own, the journal said. Science decided to retract the report itself since “a retraction signed by the authors is unlikely to be forthcoming,” the journal said in a statement.
Earlier this year, Science published an editorial expression of concern regarding the report.
This program aired on December 22, 2011. The audio for this program is not available.