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

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http://support.microsoft.com/default.aspx/kb/238573

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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