When my company was migrating from Exchange Server 5.5 to Exchange Server 2003, we ran into a problem. We have a mixed-mode domain environment that consists of Windows Server 2003 and Windows NT 4.0 machines. We use an Exchange multisite setup in which all the domains link to an Exchange server in the main site. Because of this environment, we were having problems with creating a Global Address List (GAL). We wanted to include everyone from every site in the GAL without having to migrate all the clients to Windows 2003 Active Directory (AD) and set up Active Directory Connector (ADC) agreements for all the servers in the sites.
I then found a great registry hack that lets you point your clients to any Global Catalog (GC) server so that you can use that server as the main source of directory information for those clients. In the HKEY_CURRENT_USER\Software\Microsoft\Exchange\ExchangeProvider registry subkey, you set the value named DS Server to the GC server's Fully Qualified Domain Name (FQDN).
On all our clients, we set the DS Server value to our Exchange 5.5 server, which connects to the main site. Thus, the Exchange 5.5 server acts as an SMTP gateway and directory server for users. We use an Exchange 2003 server to house the main private and public stores. We plan to use this setup until we've migrated all the sites to a centralized AD structure.