From http://blogs.technet.com/b/ucedsg/archive/2010/04/22/how-does-federated-calendar-sharing-work-in-exchange-2010.aspx

What are the benefits of Exchange 2010 Federated Sharing?

  • Easy setup of external data sharing
  • Broader reach without additional steps to setup
  • More secure with controls for admins and users

Exchange Federated Sharing is convenient

  • Sharing between two orgs or two people
  • No trusts or service accounts
  • No end user accounts and credential prompts

Exchange Federated Sharing is secure

  • Control which orgs you share with
  • Control which users can share and at what level

Exchange Federated Sharing works with online services