Take a look at this post. Quote:
It is a known bug that Visual Studio
does not sort projects alphabetically
when the projects are located inside
Solution Folders. Since I am working
on pretty large solutions (more than
20 projects for 1 solution), and I
like to order the projects via
Solution Folders, this is a very
irritating bug.
However, I accidently found the
solution to this problem. Follow the
steps below to sort your projects
alphabetically inside Solution
Folders.
Right-click on a project and select
rename (or simple select a project,
wait 1 second, and click it again or
press F2). Don’t change the name,
simply select another project.