Leading up to the release of Visual Studio 2008, I heard a couple times that solutions would be compatible between Visual Studio 2005 and Visual Studio 2008. I thought this sounded great. To me this meant that I could open a Visual Studio 2005 solution without the annoying upgrade wizard, work on my web project in the new split view, and then still be able to open it up in Visual Studio 2005. Well, not quite. When I tested this, I do in fact get the upgrade wizard. It appears that while Visual Studio 2008 does have framework targeting, the solution files are not compatible. Bummercrapola.

Fortunately, there is a fairly easy solution. First, browse to your solution file in Windows Explorer, or by using the Open Project dialog in Visual Studio. Then drag the solution file to an empty area of the window to make a copy of it. Rename it to something like '<solution name> 2005.sln'. Rename the original file to something like '<solution name> 2008.sln. Now you can open your solution in either Visual Studio 2005 or Visual Studio 2008, by selecting the correct solution file.

There is a downside to this, however. If you add or remove a project from the solution, or make other solution level changes like hiding or showing the solution node, you have to remember to make them in both files.

[Updated: 4/18/2008]

Since posting this, I have learned the other downside to this is that in order to open and build the solution after using the above steps, you will need both Visual Studio 2005 and Visual Studio 2008 installed since some of the MSBuild target references are changed.

One workaround is to use Emmet Gray's utility to switch back and forth as necessary.  You can download it from here:
http://home.hot.rr.com/graye/Articles/ProjectConverter.htm