Hey all,
I've seen a few things going around about this so I'd like to put in my .02c too.
When opening a VS2005 solution in VS2008 their will be an upgrade wizard that will show up. If you go through the wizard and 'upgrade' the solution/project here is what happens: at the top of solution you just opened the first line will change from
Microsoft Visual Studio Solution File, Format Version 9.00
to
Microsoft Visual Studio Solution File, Format Version 10.00
That's it, nothing else gets change and you *.vbproj or *.csproj are not affect or modified. So here is what I recommend you do, make a copy of the SLN file in your working folder can call it something with 2008 in it like Mysolution2008.sln. Then open with the wizard and upgrade. Then add to TFS/VSS/SVN/etc and you now can work in either 2005 or 2008 with this file or make the copy and open in notepad. Make the change I just described (9 to 10) save and then open in VS2008. Same result it just depends if you want to go through a wizard.
This is great news for most folks that are wondering if then can keep their asset working in both 2005 and 2008. It's a little awkward but at least you keep every member on your team working with the tool they have.
Oh and there is another solution, you can open a brand new solution in VS2008 and add all you proj file to it. That also works but if you have a very large SLN it might take more time to do this then copy and edit a 9 to a 10. Up to you.
Update: If you want more in-dept info please consult DJ Park's blog How to use solutions and projects between Visual Studio 2005 and 2008 and as always, open and edit SLN files outside the IDE at your own risk. Those tips have worked for me.
Cheers,
ET