Depends on how aggressively the isp shapes the traffic. Some may only do it at peak times and only limit the speed moderately, while others may just kill the traffic at all times. My isp is ntl and I have never experienced such a thing, but then I pay for the maximum service because I am a heavy user. I hear the most complaints from those who are subscribed to a minimum/light user service and think they are going to download movies all day long - not going to happen with the majority of isp's

You should always ask the isp about their "fair use policy" before you sign up and if you expect top service then pay for it!