I decided to experiment with SQL Server 2005 installing on Windows 2008 RC0. For the most part everything seems to be OK. You will be prompted that SP2 of SQL Server 2005 is required.
The Visual Studio 2005 components are not very happy.
The major issue is that Reporting Services doesn’t recognise IIS7. Oh well, all will be resolved with SQL Server 2008 which doesn’t need IIS.