That's according to CBS News, which said the inquiry is being conducted by U.S. Postal Inspectors:
A source has been told that child pornography was found in at least one of Sandusky's computers. According to the source, there is no evidence that the pornography was of his victims. There is also no mention of a ring or sharing of boys with others. CBS News investigative producer Pat Milton reports that a second source said that none of Sandusky's victims have indicated that there was shared sex with them, Sandusky and any other individual.
Also today, Radar Online reported that investigators are looking into whether Sandusky shared some of his victims with others—reviving the infamous Mark Madden rumor from last November—but based on what's in the above paragraph, CBS seems to be refuting it.
Feds probe possible Sandusky child porn ring [CBS News]