Check out this thread....
BES Router Latency?
Having the same delivery problems, and after reading this thread, I did a quick test and poll of a 130 users on my BES - the only people that experienced the delay are on Cingular. We have a mixture of Cingular, Verizon, T-Mobile, Sprint/Nextel - and ONLY Cingular customers are affected.
Spydertech