Take a trip to the forests blog:
http://blogs.technet.com/b/exchange/archive/2010/08/10/3410619.aspx
new description:
http://www.misdivision.com/blog/how-to-create-a-trust-in-windows-server-2008-r2
Cross Org Migrations
http://blogs.technet.com/b/exchange/archive/2006/11/02/3396084.aspx
more about it:
http://social.technet.microsoft.com/Forums/en/exchange2010/thread/483ba24e-a6e1-4b1e-82be-ffa0aec6fd66