It's enabled by default for workgroup machines, but disabled by default for domain joined machines. You can use group policy or edit the registry to re-enable this feature.
Enabling Windows 8 Pin Logon on a domain joined PC via Group PolicyNote: This is a Computer setting and the GPO will need to be applied to the OU containing the Computer Account.
Note: If you don't have this setting in your group policy editor, you may need to install newer versions of the .ADMX Administrative Templates.
The setting is located under Computer Configuration\Administrative Templates\System\Logon > "Turn on Pin Sign-In"
Enabling Windows 8 Pin Logon on a domain joined PC via the RegistryTo enable this via the registry create the following value.
Value Type: REG_DWORD
Value Name: AllowDomainPINLogon
A restart is required for the settings to take effect. Unfortunately, I have not found a knob that lets you control the minimum complexity or length of the PIN. Yet.
To configure the PIN login, open the start menu and search for "Create or Change a PIN" under the settings menu.