When Apple introduced iOS 5 at this year’s WWDC, one of the key features demonstrated was the ability for iOS devices to go PC/Cable-free. Activation as well as updates will be available over-the-air without the need to connect to a computer via cables.
While Apple mentioned that updates to iOS 5 will be Delta updates, thus smaller and easier to download (containing only the changes instead of a complete update image), they never said anything specific about updating via 3G. 9to5mac has looked into the iOS 5 code to find that some OTA updates to iOS 5 will indeed be supported via 3G.
However, the same code implies that not all updates will be available via 3G, some requiring WiFi. We tend to believe that small Delta updates will be allowed via Data Connection but larger version number bumps (in a larger container file) will most probably require WiFi to improve speed, reliability and take the load off carriers. iOS 5 will also check for battery level before starting an update which is a normal and expected feature to protect the hardware.