Microsoft released the official 8.1 update for Windows 8 today.
During the upgrade process, Windows will ask you for an online Microsoft account information, even if you opted to use a local (offline) account when you initially installed Windows. During the original install of Windows 8 you have an option to use a local (offline) account, but this option seems to have been removed with 8.1.
Here is one trick to skip the use of an online Microsoft Windows 8.1 should you choose to do so. When Windows 8.1 asks for your Microsoft account information (including the option to create one), simply enter fictitious or bad information. In other words, make up a random username and password.
After you attempt to log in with this bad information, you will then have the option to use a local account.