Although unconfirmed, it appears that it will be up to an individual carrier to decide whether or not it will allow an OTA update to be allowed through its precious pipeline. One interesting feature that Apple has set up allows the the update process to start only if the user has enough battery life and storage to handle the entire download. The Cupertino based firm wants you to have 500MB in open storage for each "Delta" update. A "Delta" update is one where only the changes and new code are sent instead of the entire program.
While Apple has not officially made any announcement about updating over 3G, at WWDC Apple's Senior VP of iPhone software, Scott Forstall, had mentioned that no longer would a computer hookup be required for an iOS update. But nothing was said about whether such an OTA update would require Wi-Fi or could use 3G even though the slide on the screen at the time showed a 3G powered Apple iPhone. And while we still await official word from Cupertino, at least we now have an idea about which OTA updates can be downloaded using 3G.
Some smaller "point" OTA updates can now be installed on the Apple iPhone over a 3G connection