I've got T-Mobile, Nextel, Sprint, and ATT&T running on our BES. We purchased the BES through AT&T and got the upgrade to 4.0 from T-Mobile. So, yes, you can mix and match all you want and it won't harm a thing.
Current - 8700g TMO
Previous - 7280 AWS, 7730 TMO, 7290 TMO
Long ago - 950 model (still have it)