I am an audio technician, they actually did lip sync extremely well, although there is no way they would jump up and down like that and sound so good....
biggest give away, was when they were getting interviewed afterward, did you hear the quality of those headset microphones?
in studios when recording they use pop sheilds, (view image below)
http://farm1.static.flickr.com/163/356844075_bd014f2b7c.jpg?v=0
what we heard was a classic example of how a mic sounds without one (when they got interviewed at the end)
Also it was too obvious how it sounded, the sound quality was far superior to what anyone could get in a live performance, unless the sound engineer has GOD like tools behind the desk.......
recording sound and live sound is so much different.....i think we have established this.......
Melodyne is software which is pitch correction software for vocalists, they were even using this on the recording, i can spot it a mile off....
example here of how it works...
http://uk.youtube.com/watch?v=8zeLKMHxbN8
ok well i hope this clears things up for you, YES they were miming.....quite sad actually.....this is were commercialism and a true talent show collide in such a way that commercialism will always win....