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This should help:
http://technet.microsoft.com/en-us/library/gg712701.aspx#Plan_Com_X_Forest
http://blogs.technet.com/b/neilp/archive/2012/08/24/cross-forest-support-in-configmgr-2012-part-3-deploying-site-server-site-systems-in-an-untrusted-forest.aspx
