I have a glass top stove, and I use all kinds of pots and pans on it (I've got a little bit of this and a little bit of that): aluminum, t-fal, adonized, stainless steel, and yes, even cast iron (its a skillet, probably 10 or 11" size). I've had my stove for almost 9 years, and there's no scratches on it. I've never had any problems at all.
I clean it with the Weinmann (?) or CremaBryte cleaner and a scotch brite pad. If I get something sugary or really burned on brown rings, I use the Crema Brite, let it sit until it starts drying out and then scrape the surface with a razor blade. Again, I've got no scratches and never had a problem.