From an MSN article which says Shania Twain was not lip synching at the Super Bowl (which I still believe she completely was!):

Other bands use a different mix of taped and live elements, depending on the nature of the show. At a dance-heavy concert where the lead singer does exhausting choreography, we might hear a tape of the lead vocal track. At an event like the Super Bowl, where sound engineers have five minutes (rather than the usual six or eight hours) to set up, bands are more likely to rely on tape. During No Doubt and Sting's halftime sets, we were also hearing live vocals and canned instrumentals. Last year, when U2 played, we heard both Bono's voice and the Edge's guitar live, though the rhythm section was prerecorded. Maggie