There has been a bit of media coverage in Australia surrounding the Britney Spears tour. For those that don't know, Britney lip synchs practically 98% of her show (I have heard that she sings some of her ballads live but I have never been to her concert so I don't know 100%)
The issue that has been raised is the amount of money people are outlaying to see a live show ($200 upwards in many cases) when in fact it is lip synched.
I know that most artists these days are using some kind of "help" with their "live" performances. A lot use vocal boosting (where they sing over the top of their pre recorded track), auto tuning (where bum notes can be eliminated live) and a pre record of them singing in rehearsal live then played back for the show. So there are lots of ways to cheat and appear like you are singing live without having to mime an entire show.
My feelings regarding lip synching are echoed by none other than Sir Elton John in the clip below at 0:55