One of the more common questions we get from customers with multi-forest Exchange implementations is "How do I provision my Exchange mailboxes?" Many customers provision mailboxes using various third-party tools, or with tools from Microsoft such as Microsoft Identity Lifecycle Manager 2007 or Microsoft ForeFront Identity Manager 2010, or with free tools from Microsoft. There are always questions about which version of tool works with which version of Exchange and how it all plugs together.
Aung Oo coauthored (with Rob Ward and Henna Kermani) a great article that cuts through all of the confusion in a very clear manner. If you need to provision Exchange recipients, I highly recommend this article. Head over to TechNet Magazine to read Exchange Provisioning using ILM 2007 and FIM 2010.