When looking for a solution on how to synchronise our Exchange GALs between two seperate forests, I came across this Microsoft feature pack – which is freely available to download from their website.

What IIFP does according to Microsoft: “Identity Integration Feature Pack 1a for Microsoft® Windows Serverâ„¢ Active Directory® manages identities and coordinates user details across Microsoft Active Directory, Active Directory Application Mode (ADAM), Microsoft Exchange 2000 Server, and Exchange Server 2003 implementations. ”

The download link:
IIFP Download

There’s only one catch: you need an enterprise version of Windows 2003 Server to be able to install the feature pack. A little birdie tells me that if you use Orca from the Windows 2003 Server SDK, you can modify the installation requirements to get it working on a non-enterprise version of Windows 2003 Server.

Here’s a tutorial from MSExchange.org on GAL synchronisation:
http://www.msexchange.org/pages/article_p.asp?id=668

I couldn’t end up getting it to work because of domain naming issues between the two domains (one is named as a parent of the other one without actually being the AD parent domain), but it looks like it could be a very useful tool.