Here you have a bug that can happen navigating programatically the nodes of the Solution Explorer of VS 2005 if you have solution folders and their nodes are not expanded by hand previously:

BUG: EnvDTE.UIHierarchyItems does not work correctly in Visual Studio 2005 if Solution Explorer nodes are not opened previously
http://www.mztools.com/articles/2007/MZ021.htm

I am not sure if Microsoft will consider a bug or a side-effect of the optimizations made in VS 2005 to open the solutions faster, but, for me, it is a bug and I reported it here:

https://connect.microsoft.com/VisualStudio/feedback/ViewFeedback.aspx?FeedbackID=269526

You can vote for it if you consider it a bug too.