It's the first time a major college championship has been vacated in major college sports since polling began - all because of the shenanigans connected to former Heisman Trophy winner Reggie Bush. From the BCS announcement (via USA Today):
"The BCS arrangement crowns a national champion, and the BCS games are showcase events for post-season football," executive director Bill Hancock said in a statement. "One of the best ways of ensuring that they remain so is for us to foster full compliance with NCAA rules. Accordingly, in keeping with the NCAA's recent action, USC's appearances are being vacated. This action reflects the scope of the BCS arrangement and is consistent with the NCAA's approach when it subsequently discovers infractions by institutions whose teams have played in NCAA championship events."