There is a feedback Request already placed at Connect.Microsoft.com so, you will have to wait for the permanent solution.
http://connect.microsoft.com/VisualStudio/feedback/details/468874/solution-explorer-project-sort-order (replaced with Wayback Machine link as original was removed)
Temporary workarounds are available but not good enough I feel. One of them is:
Press F2 to rename a project, then press Enter without changing the name. This temporarily sorts the projects within that folder.
(As per http://social.msdn.microsoft.com/Forums/en-US/vssetup/thread/79d46a69-386e-4ad8-b9cb-b00d4252495b/?prof=required )
Or use http://solutionsorter.codeplex.com/ to alter SLN for permanent fix if your solution folders are not nested (it ate my solution)
But this will not work if you close and open the solution.
If you use Resharper, you can press
the short cut CTRL + N to find out
files in solution and CTRL + F12 to
find out methods in the file.