GAL Segmentation has not been supported in Exchange 2010.  Just announced on the Exchange Team Blog, GAL Segmentation will now be supported in SP2 for Exchange 2010 which is scheduled in the second half of 2011. 

A few snippets from the above referenced link:

In order to provide transparency in these early stages of development, our intentions for this feature are:

  • This feature is not intended to enable the On-Premises configuration of Exchange 2010 to be used in place of the multi-tenant enabled version of the Exchange 2010.
  • Our intended audiences for this feature are organizations that:
    • Require some form of sub-divided address book or who wish to create several ‘virtual’ organizations within a single Exchange Organization.
    • Enable users to share some resources between these segmented user populations
    • Seek to control which objects are visible to a user when they open their address book picker.

The way we intend to deliver this feature is by using an "Address Book Policy" assignment model, rather than continuing with the ACL based GAL Segmentation concept we previously provided. This approach is not intended to provide complete tenant isolation as is provided in the Hosting mode available in Exchange 2010 SP1, rather it is intended to allow an administrator to grant access to certain views of the GAL rather than restricting views.

So stay tuned…