Multiple Forests running Exchange
I recently answered a questions about running two Exchange Orgs after a merger.
The company required GAL sync, Public Folder replication and Free/Busy sync.
Here are a couple of useful links on the same topic
http://www.microsoft.com/technet/prodtechnol/exchange/Guides/PlanE2k3MsgSys/e477a729-e0d9-44e3-816c-f3fe39e6fa36.mspx?mfr=true
http://support.microsoft.com/default.aspx/kb/238573
The first link details the features available and what is required to implement them and the second is about the InterOrg Replication tool.
Cheers
Nathan