Well... there are some Microsoft customers who would like to have two different forests. They'll use one forest to deploy resources like LCS, OCS, Exchange etc.. and they will use other forest to host users. This is fairly simple using any identity
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Read the complete post at http://blogs.technet.com/ramo/archive/2008/06/29/inter-forest-lcs-ocs-deployment-without-identity-integration-application-miis-iifp-etc.aspx