The SQL Server 2008 Upgrade Technical Reference Guide is now available.
This 490-page white paper covers the essential phases and steps to upgrade existing instances of SQL Server 2000 and 2005 to SQL Server 2008 by using best practices. These include preparation tasks, upgrade tasks, and post-upgrade tasks. It is intended to be a supplement to SQL Server 2008 Books Online.
In addition to the Technical Reference Guide, a companion list of Upgrade-related resources is also available
SQL Server 2008 Upgrade Technical Reference Guide
Resources for Upgrading to SQL Server 2008