Update: There's a bug in iOS 8.0.1. After updating, many users are reporting loss of all cellular service and Touch ID. I can verify this personally. We'll be posting instructions on how to revert to iOS 8.0 soon. In the meantime, do not update!
Less than a week after shipping iOS 8.0, the first update to the new mobile OS -- containing improvements and bug fixes -- is now available.
According to the release notes seen above, the release fixes a known bug that kept HealthKit apps from being available on the App Store at the initial release. For users of the new iPhone 6 and 6 Plus, the update improves the reliability of the Reachability feature. That feature moves top of screen features to the bottom of the phone when the Home button is double-tapped.
The update is available as an OTA (over-the-air) download that can be access from Settings > General > Software Update. As usual, information on the security content of the update will be available at http://support.apple.com/kb/HT1222.