I have always heard that shoes with the wheels in them (Heelyz or whatever they call them) are bad for you're knees. Now since I know we have plenty of people here at APK that know about this stuff, I was wondering how? And why?

If I had to take an educated guess, I'd go with the altered gait.

Just compare it to high heels.
As far as when you're actually rolling on them, I don't think there's any significant damage, unless you slip or something.
But when you're just walking in them, your stride is definitely different, like high heels.