As of version 10.x of ArcGIS it is possible to borrow licenses. In large organisations where ArcGIS Desktop is used on Desktops, notebooks and Citrix one would prevent users on Citrix to borrow a license. Curruntly this is possible using the "EXCLUDE_ENTITLEMENT" option with a HOST_GROUP. The drawback is that every single Citrix worker has to be defined. It would be nice it the use of wildcards was allowed to define a range of workers without the need to exactly specifing every single citrix worker.