Does Blackberry Enterprise Server support users from multiple carriers at the same time? We are currently using Nextel and would like to add some users from Cingular. IS this possible?

Yes, it is possible, right out of the box.

As murpheous said, yes. I currently support users on Cingular/AT&T, Sprint, Nextel, T-Mobile, Verison, and Alltel just to name a few. They all work on the same BES server no problem. Some carriers are easier to deal with than others, but not because of BES issues.