mimed, very very definately mimed.
Worth noting though that even if your mymeing the mic is still 'on', hence how you can make noises/talk/sing allong to bits of it. Usualy there's a pre agreed signal to the sound desk (mic in other hand, lift finger off mic, grab coat etc) when your about to adlib etc.
Allegedly, the only big band that doesn't myme is Coldplay, smaller bands (and festivals) tend to be less mymed as the sound engineer tends to be from the venue not traveling with the band.