When updating to SQL server 2008, running the update advisor is a good start. However, there are a few issues that the advisor does not properly identify. The purpose of this article is to list these types of issues. As you find more, please add to this document.
Prior to starting an upgrade, make sure that the existing instances have the newest service pack applied. At this time, SQL Server 2000 sp4 and SQL Server 2005 sp2 are the latest service packs. If you install a service pack, you will have to reboot the server prior to upgrading to SQL Server 2008.