I was originally planning on upgrading my Blackberry 8700 to a T-Mobile Dash when it was released.
I had complained about a leak in my system. As Tony commented, it is possible to have BES as well as multiple BIS accounts on the same device. Though I had mail from both accounts delivered to the device, it wasn't set up properly due to the security policy. The problems with synchronization and replying from the proper account was a result of this policy, not a limitation of the device itself.
Not only have I plugged the leak, but I also managed to completely consolidate devices - something that would not have been possible with the Dash.
Thus, I've decided to stick with the Blackberry 8700 for the time being. If anything, I may consider the Pearl.