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