To be honest, it does not really require SQL Server 2008, only the SQL Server 2008 SMO assemblies. However, if you try to target a SQL Server 2008 from a SQL Server 2005 without .NET 3.5 SP1 and 10.0 SMO the results are incomplete. Its the same problem you have with trying to use DMO on a 2005 instance.The backward compatibility scenario is a much better experience. So I decided it was better guidance to point yoou toward SQL Server 2008. I think it is spelled out a little better in the "Installing SQL Clue" help file.
I added that installer check box page with the SQL Server 2008 requirement message last week. The check list is there as a reminder. It helps eliminate confusion about why the install fails when it fails. It also allows you to bail on the installation before anything happens.
The true prerequisites - those checked in the Visual Studio setup project - are Windows Installer 3.1, .NET 3.5 SP1, ReportViewer 2008, and [a minimum of] SQL Server 2005 Express SP2. These can all be downloaded directly from Microsoft (at no cost).
SQLClue is a great first SQL Server 2008 box for the DBA. It is an application fully under the control of the DBA and it uses features new with 2008. While the stuff installs, configures and runs nicely, you get an opportunity to get some time behind the wheel in a low stress but highly useful way before the shop adds a line of business 2008 instance.