I used to do this at dnmanager on my domains. A lot of people ran round wailing and gnashing their teeth with cries of 'that's not allowed! google will ban your account, kill your firstborn etc'. However outside of that (google never cared) it earned sod all. Now I just like clean crisp parking pages (at 123parked of course) and, although I don't have any evidence at all, I think that throwing adsense on a parked page likely cheapens it.