Here's how. If you installed the 1.0 SDK to Visual Studio 2005, a directory was created under Visual Studio 2005\Templates\ProjectTemplates\Visual C# named Silverlight (which contains a zip file). You can copy and paste that entire directory (with its zip file) into Visual Studio 2008\Templates\ProjectTemplates\Visual C# as is and it just works.
Doing so will give you the project template in Visual Studio 2008 to create Silverlight 1.0 projects
(Click on image to see it at correct size)
NB: This is the first Tip of the Day, but not the last, on Programming Silverlight. The idea was inspired by the incredibly useful Visual Studio Tip of the Day offered by Sara Ford.
Each Tip of the Day will be tagged, and will have a subject beginning "Did you know…"