I have had this happen to me also, and I believe that it is due to a battery-conservation feature. I think it turns off to conserve battery power. Try to leave your unit plugged in for, say, overnight and see if it still disconnects your wireless.
This is interesting! I use a 7250 on Verizon. I have my device set to shut-down automatically at midnight and turn back on again at 5a. I have never had an issue with the wireless not coming up as a result of the device turning back on. The one difference is that I am not using a BES, but I can't imagine how that would affect this situation.
Now you can call me STORMin' Norman, except my name isn't Norman.
When I opened the cover this morning around 5:50am, the unit was still on and the wireless function was off. At 7am when it should have automatically turned on, no change in status and the wireless adapter was still off.