Just open your 2005 project in 2008 and allow it to upgrade your solution/projects. You may get a dialog asking you to update CLR versions, just say no.
Once you've got a working 2008 solution, copy the *.sln file to another name (I call my copy *.2005.sln), open the copied *.sln file in notepad, or your favourite text editor. Find "Format Version 10.0" on the 2nd line, and change to "Format Version 9.0". Save the file.
That's it, your upgraded solution file works in VS 2008, and the one you copied and changed works in VS 2005.
There's one extra step if you've got Web Application Projects.
In the Web Application's vbproj or csproj file, find the line:
<Import Project="$(MSBuildExtensionsPath)\Microsoft\VisualStudio\v9.0\WebApplications\Microsoft.WebApplications.targets" Condition="" />
and replace that one line with these two:
<Import Project="$(MSBuildExtensionsPath)\Microsoft\VisualStudio\v8.0\WebApplications\Microsoft.WebApplications.targets" Condition="'$(Solutions.VSVersion)' == '8.0'" />
<Import Project="$(MSBuildExtensionsPath)\Microsoft\VisualStudio\v9.0\WebApplications\Microsoft.WebApplications.targets" Condition="'$(Solutions.VSVersion)' == '9.0'" />
Those two lines will conditionally select the correct MSBuild path depending on the version of VS used.