Update 2:20 PM EST: Apple pulled the iOS 8.0.1 update.
PSA: Do not install the first update to iOS 8.
Early reports indicate it will leave your phone crippled, without access to your cellular network and Touch ID. Rebooting or restoring the handset did not seem to fix the issue, according to discussions on Twitter, but restoring to iOS 8.0 does successfully revert the problems.
More on the iPhone 6:Duh: Of Course the iPhone 6 Plus Can Bend in Your PocketApple iOS 8 vs. iOS 7: A Visual Tour of the Biggest UpdatesThe 8.0.1 update addresses the HealthKit bug that plagued the initial launch of health-tracking apps in iOS 8. It also addresses other issues like access to photos in Photo Library (some apps were having problems with this) and the deselection of third party keyboards after a user enters their passcode.
Further reports seem to indicate this is mostly affecting iPhone 6 and 6 Plus handsets, and at least one iOS 8 user was able to solve the problem by going into Device Firmware Upgrade mode to restore his device.
So while we should start seeing HealthKit-enabled apps landing in the App Store over the next few days, you potentially won't be able to use them over your cellular network, or log into your device using Touch ID. Hopefully Apple will roll this update back swiftly. In the meantime, Do Not Update Your iOS Device.
