I came across a fantastic piece of information for those who are looking to upgrade their existing SQL Server 2000 or 2005 instances to SQL Server 2008.  I know from past experience that it is no fun flying into this type of upgrade blind so this is a must read before jumping into the upgrade analysis. 

All of the good juicy details about the upgrade process are documented with great detail in the SQL Server 2008 Upgrade Technical Reference Guide.  This is no light read, it has an amazing amount of detail, but the nice part is the index and table of contents allow for easy navigation based on your particular upgrade scenario.

In addition, I found that the MSDN site had the document online in case you're not sitting with the doc in front of you.  It can be found here.