Nobody in their right mind would sell T1 to every customer and do 1000:1 overcommit of their transit links without some kind of bandwidth shaping. Since that shaping is part of the engineering plan, the marketroids will have to find some way to come clean about it in the advertising glossies.
We've got 40 T1 ADSL customers coming into a single 1.1Mb SMDS connection Traffic is around %3 to %5 on average with occasional spikes to %50 or %100 for short periods. Im guessing this behaviour is due to a single PC at the customers side -- limitations on how much traffic a single user can generate - how much bandwidth a single stream uses. With a DS3 you could get perhaps 5,000 to 10,000 "T1" customers is a rough guess. Look at @Home's model for example. Stb