Yep, I have a 17" flat screen TV and my boys will occasionally sit on top of it even though it teeters frantically and I come charging over to scoop the culprit off.
They've also sent it flying about half a dozen times when they're playing 'chase' round the flat.
So far the worst that's happend is that the back's fallen off and I've had to put it back together again, but I do tend to screech a bit if they're on the rampage and head in the TV's direction.
Sometimes I love the fact that Singas stay young at heart, sometimes I wish they'd 'mature' just a little...