I get this once in a while as well. Although, now that I think of it, I don't think this has happened since the firmware update.
What exactly happens - Mobile Network setting goes into Sleep mode. If you take a look at the setting when this happens. Settings, Wireless Controls, "Mobile Network." Instead of it saying 'Connected,' it will show Standby.
I figured this was happening to me because I have the following setting turned OFF:
Settings, Wireless Controls, Mobile Network Settings, Enable always-on mobile.
My work around has been to add a shortcut to the "Mobile Network" setting and I wake it from Standby by turning it off and then on again. Probably not the best solution but I've been busy the past few weeks. Too busy to look for help.
Perhaps someone could shed some light.