Upgrade to Win2008 Server Active Directory

We have a 2 forest Win2003 domain (with root forest trust) and are looking to upgrade to a Windows 2008 Active Directory single forest domain.
What would be the best way to migrate from 2 forest to a 1 forest domain? We also have Exchange 2003 and will be moving to Exchange 2010.
Is there a guideline from Microsoft or a best practice approach?