Limit the People Picker in SharePoint 2010 to a Specific OU or Domain

STSADM: stsadm -o setsiteuseraccountdirectorypath -path "OU=Sales,DC=ContosoCorp,DC=local" –url http://ServerName

Powershell:
Set-SPSite -Identity http://portal.contoso.com/sites/US -UserAccountDirectoryPath "OU=US,OU=SharePoint,DC=Contoso,DC=com"