I agree with Riven.
I don't ration DS's screen time at the moment, and he is Wii-crazy. He watches a bit of TV before school (provided he's dressed and ready), plays at least an hour of Wii when he gets home, sometimes plays on websites on my computer, and sometimes watches a bit more TV before bed. He does loads of Wii with his dad, and quite a bit with me (I enjoy a lot of the games too).
So the Wii is The Big Thing at the moment. (He does have playdates with friends, goes swimming, cycling and to drama too, so it's not the only thing he's doing.)
I am hoping that he will learn to self-regulate, because that is what I like the idea of. And the more I restrict, the more it becomes the forbidden fruit.
TBH, it's also a real help because it's just DS and me, so it's handy entertainment if I'm busy and there's no one else to play with.
However, since borrowing the Wii, DS has occasionally said that playing with "normal" toys is boring, which has worried me a bit. When he has to come off (to eat/go to bed/do chores/do some exercise), he can be arsy, and struggle to adjust to bog standard play again. But when he's not on it and gets back into the swing of "normal" play, his imagination's as vivid as ever, and I can see that the games are inspiring his play. I have been amazed at the problem-solving and strategising he has got his head round on some of the Wii games.
I'm mindful of the possibility that DS is getting a bit addicted, but for now, I'm continuing with my self-regulation experiment, in the hope it's a fleeting obsession. (We haven't had it that long.)
DS's five, BTW.