I watched it and you could tell he had a backing track. But his songs are layered, so unless he admits to something we’ll never know.

The Real Slim Shady?
After last week’s Ashlee Simpson debacle on Saturday Night Live, you’d think it would be a while before anyone would dare to lip-synch on the ”live” show again. But this past weekend, amid several comedy bits lampooning the accidental revelation of Simpson’s vocal backing track, musical guest Eminem performed his song ”Mosh” to the apparent accompaniment of a vocal track. As clips of the performance that have surfaced on the Web indicate, there was at least one moment during the song where Eminem dropped his microphone away from his face while his voice continued to boom through the speakers.
A spokesman for Interscope, the rapper’s label, acknowledged that Eminem did perform with a backing track but dismissed comparisons to Simpson. ”This is not that big a deal,” Dennis Dennehy told EW.com. ”This happens all the time. He has a double vocal track on the song. When you hear the actual [recording of the ] song, there’s two vocals doing the lyrics. For Saturday Night Live, what he wanted to do was to use one vocal track to back up his live vocals.” In any case, Dennehy said, rappers play by different rules than rock singers like Simpson. ”It’s hip-hop; he can sample his own voice if he wants.”