How to Create Routing Group Connectors from Exchange 2007 to Exchange Server 2003
New-RoutingGroupConnector -Name "Interop RGC" -SourceTransportServers "Ex2007Hub1.contoso.com" -TargetTransportServers "Ex2003BH1.contoso.com" -Cost 100 -Bidirectional $true -PublicFolderReferralsEnabled $true