Microsoft Knowledge Base Article 302430 contains the following summary:

You can use group policies to assign or publish software to users or computers in a domain, and it is useful to be able to deploy software based on group membership. Group Policy Objects (GPOs) are normally applied only to members of organizational units (OUs)to which the GPO is linked. Because users cannot be located in several OUs at one time, it is necessary to be able to apply group policies outside of the boundaries of OUs. This article describes how to have your software deployment policy applied to users who are not in a respective OU.