Yup,
Incase you hadn't noticed, the artists always mime their instruments at T4OTB, but most of them do actually sing live. I believe if they (at least) sing live, they are classed as a 'live' act.
This is due to them trying to cram as many bands in as possible. You cannot physically change amps, drums, linechecks etc in the space of an ad break!
Some of them did it better than others; Manics were very good, as were the Kaiser Chiefs (who could have been playing as they were the last band on), Super Furry Animals seemed to mime everything.
Hats off to the new 4music stage artists. Each band played fully live, which was great. Notice how it took about 20 mins between bands, this is why.
I think its amazing how the 'bigger bands' mime on the main stage, yet the so-called smaller bands play properly on the smaller stage.

Hope this clears things up!
Gaz
Neo-Geek