I just used the "tech chat" online feature offered by Cingular. The rep said that leaving the BB connected via USB to you computer for any amount of time is OK.
Interestingly, he said that when using the wall charger, you should unplug it after 5 hours. This is info they have from RIM, anything more than that will reduce the overall battery life.
In addition, he did say that its very common for people to leave their BB's connected to the wall charger overnight and that it does not seem to present a problem. I found it refreshing that they are candid and do not seem to be prone to place blame on the customer.
In my case a replacement battery is being sent at no charge to me under warranty.