If you're unaware of the current market share difference between Windows 8 and Windows 8.1, prepare to be shocked. A little under half of Windows 8.x users still use Windows 8.0, which means they haven't taken advantage of that FREE Windows 8.1 upgrade via the Windows Store, and who can blame them considering the upgrade from the Store was an atrocious experience. Microsoft is aware of this, and in an attempt to get more users to update, has started testing delivering the update via Windows Update.
Microsoft is currently testing this new upgrade method with Windows RT users, and if proven successful will begin using the method on x86/64 based PC's too. This 'pilot program' will allow Microsoft to push Windows 8.1 via Windows Update, much like Windows 8.1 Update, that will install over Windows 8 easily and in the background.
What does this mean? Well, hopefully it means Windows 8.0 users will be updated to Windows 8.1 without needing to do anything. It'll happen in the background with Windows Update, and Windows will notify you when you need to restart. This restart will execute the Windows 8.1 setup, and before you know it the user will have 8.1 installed on their 8.0 hardware.
For now, this feature is only available for Windows RT users, but as said above if proven to be successful, Microsoft will roll out the feature for every Windows 8.0 user. Enterprise users we imagine won't have to deal with this, because if they're still using Windows 8.0, that's probably due to hardware and software requirements.