Can I create an Exchange Server organization that spans multiple Active Directory (AD) forests?
Not exactly. The configuration naming context (NC) confines the Exchange organization to one forest. However, Windows Server 2003 lets you create cross-forest trusts that work in much the same way as Windows NT 4.0's cross-domain trusts. With these cross-forest trusts, you can create a resource forest that holds all your accounts, then create a separate forest for your Exchange organization.
Alternatively, you can create separate forests and install Exchange in each of them. However, doing so only scratches the surface of the overall effort required to get a multi-Exchange-organization messaging system to work. You'll still have to synchronize the Global Address List (GAL), perhaps with the new Galsync utility; you'll have to grant the proper public folder rights to users from the various forests; and you'll have to make calendaring work. For these reasons, you'll spend a lot more time and money on a multiple-forest setup than you would for an equivalently sized single-forest deployment. Before you make that investment, be sure you'll be getting enough business value to make such a setup worth your while.