Exchange 2010 provides an easier way of sharing free/busy, calendar and contacts with recipients in another forest with the implementation of federated sharing. Federated Sharing uses Microsoft Federation Gateway, an identity service that runs in the cloud as the trusted middle man. Organizations wanting to use federation need to establish a federation trust with the Microsoft Federation Gateway.
To establish a federation trust between Exchange 2010 organization and Microsoft Federation Gateway, you must use an X.509 SSL certificate from a certification authority trusted by the gateway. Only few CAs are trusted by federation gateway at the moment & hence I urge you to take a look at the list during the planning phase of federated sharing.
Get the list of trusted CAs here