In 4.1 you could also install the database notification system on remote SQL which reduces WAN traffic. Also, as long as your latency is consistently under 100ms to that box; it seems my MAGT logs complain about the SQL connection when it gets higher than that.
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BESX 4.1.7 on Exchange 2003: 65 Devices
BESX 5.0.3 on Exchange 2003: 2007 Devices
We have a DR/HA BES and SQL DR box in a remote location and the ping time is under 40ms (dark fibre!) so as long as you have a good pipe you will be fine .. ping your mailbox server .. I believe RIM wants this around 50ms or under .. higher the number the longer it takes your email to deliver on the BB
This scenario works. You need a latency of maximum 250ms and 100bytes/sec bandwith. This is an info from an official RIM presenation given at last years CWTE.