I went to the Concert for America in Washington D.C. and probably a third of the national acts sang to tracks. Puff Daddy and Billy Gilman come to mind. Gilman, the nice little kid who sang Oklahoma and One Voice, sang to a CD music track. Unfortunately it skipped (in front of 50,000 people.) The kid never lost his cool and told the engineer over the mic to stop the CD and that he would finish it accapella. He brought down the house. It was awesome. Totally in key, moving, and he hit notes in the stratosphere.
Sadly I heard his label dumped him when his voice changed.