UPDATE: Looks like Apple's pulled iOS 8.0.1, so iTunes will restore you to 8.0.0 instead.
Apple has just released iOS 8.0.1, a bug fix update for iOS 8. Unfortunately, it's left some iPhone 6 and [iPhone 6 Plus with Touch ID and cellular connection issues. Reboots don't seem to fix it. It's possible Apple can fix and re-issue the update quickly, or a workaround will be found shortly, but if you've already updated, and you're affected, here's what you need to do:
Full story from the iMore Blog...