The official statement behind the whole Def Leppard rumor has been put to rest. According to Activision
The Guitar Hero III: Legends of Rock Def Leppard song pack – featuring “Nine Lives”, “Rock Of Ages” and “Photograph” will be available online for Xbox 360 and PlayStation 3 (North America only) on the 24th April 2008 and for European PlayStation 3 owners from 8th May 2008.