Some serious issues have been affecting a portion of people upgrading devices to iOS 4, according to Apple's support forums. Perhaps the most vital involves cellular data, which some iPhone owners have reported as simply not working, whether on 3G or EDGE. The issue may be tied to carrier updates for iOS 4, but in any case some users are reporting that it may be necessary to reset settings, or even restore a device to a fresh state as opposed to a backup.
