There seem to be a lot of bits and pieces of the latest, GDR3 update for Windows Phone emerging as more people get to looking around the system settings. Either after installing the Dev Preview, or if you are lucky and have a HTC 8X on AT&T, or the international unlocked version.
Controlling data use has always been a contentious issue with any smart phone, and the fact that Windows Phone allows apps to do a lot in the background, as in updating, and downloading. SO depending on how many apps you have installed and what they do, your data plan could take a decent hit. Prior to GDR3 there was no way, no central setting to restrict background data downloads, you had to do it by app via the settings, and some apps could not be blocked.
As small as this may seem, it now gives a simple, single location to quickly limit background tasks. Apparently it is not the same on all devices, on phones that have Data Sense, the settings appear there, whilst on my HTC they are in network settings. I also cannot ascertain whether apps that could not be blocked from running in the background before are actually affected by this, but I assume this is a on or off solution for background tasks. Seems you will at least need to have OS version 8.0.10501.127 for this to be available.