"Good news for those who travel a lot, iOS 5′s over-the-air update feature will work over your carrier’s 3G network, not just Wi-Fi. That means easy and PC-free updates, wherever you are.
Hopefully, for users on capped data plans, these over-the-air updates won’t be too huge. Especially considering Apple is making all iOS 5 updates 'delta updates,' which means you only download what’s new, and not an entirely new iOS like we’ve done up until now."
