To the "uptight"-ers comments above:
You realise that there ARE rules against this sort of thing, and that if you condone them breaking this rule- "it's not hurting anyone" you're pretty much condoning Phorm and BT's secret trials- they didn't hurt anyone either.
Were the complained about instances part of a song? Or just addressing the crowd/viewers? And if it's part of the song, WHY THE HELL DIDN'T THE BEEB HAVE A BLEEP MACHINE GUY STANDING BY?! They can get a hold of the song off their own radio station and listen to it to find out when (or if) there's going to be inappropriate language present.
And no, I'm not particularly uptight- especially about swearing. I just think that the Watershed should remain- keep a reasonably safe environment on TV for kids. Not sanitised, but just "safe"; by all means have swearing. But bleep it out. So there's an acknowledgement of the swearing and that people do it- rather than hiding the kids from it completely- but it's made "safer" and doesn't annoy as much as just muting the sound when someone swears.
Even better, with modern interactive TV the bleeps could be held on a separate audio channel so if you wanted the unfiltered sound you get it.
Though it does beg the question, do they subtitle these events? And if so, do they fuzz out the offending lyrics?