It looks like SQL 2008 may have a slight dependency issue. If you have already installed Visual Studio 2008, you will be blocked from installing SQL 2008 until you install Visual Studio SP1. The problem is that Visual Studio SP1 is not released yet. Our guys came in ahead of schedule and they still get no respect.
Not to worry, Visual Studio 2008 SP1 should be out very soon (think days, not weeks) and this problem goes away.
Microsoft actually documented this issue here:
Visual Studio 2008 SP1 may be required for SQL 2008 Installations
UPDATE: It's Here