Like Silk.  I had an instance of the full imprint of SQL Server 2000 SP4 (Database Engine, Analysis Services, Reporting Services, etc.).  I just ran through the setup and it worked wonderfully.  It detected the installed products and versions and asked me what I wanted to do with them.   When attempting to upgrade, select the SQL Server 2005 instance as the default instance.  It notices that you already have a default instance and will prompt to upgrade it.  Next, I'm going to try it on my Virtual Machine which has a the August CTP bits.