Back in VS.NET 2002/2003 a solution could contain only up to two folders, which were special: the Solution Items folder and the Miscellaneous Files folder (if you configured a setting in the Options window to show it), and the automation model (EnvDTE) contained their well-known GUIDs to identify them (EnvDTE.Constants.vsProjectKindSolutionItems, EnvDTE.Constants.vsProjectKindMisc).

VS 2005 introduced a new concept, the “solution folders”, that could contain projects, and you could create lot of them, and you had a new well-known GUID (EnvDTE80.ProjectKinds.vsProjectKindSolutionFolder As String = “{66A26720-8FB5-11D2-AA7E-00C04F688DDE}” to identify such folders, and guess what, alas, it happens that the old Solution Items folder is just a “solution folder” in the new IDE, indistinguishable from the others but by the name, so the GUID cannot identify it and then you can only rely on the name “Solution Items” to distingush them (because the IDE doesn’t allow you to create two solution folders with the same name), and the Project.UniqueName includes a GUID too (such as “Solution Items{24571E2E-0233-4835-9C48-80E8D75EC95E}”) to make them truly unique I guess (just in case you nest them, because as I said, the IDE doesn’t allow two folders with the same name with the same parent, but it does with different parents).

So, in VS 2005, there is no really a unique “Solution Items” folder, you could rename it, or have tons of them with files, even nested, but if you really want to locate and use it as you could in VS.NET 2002/2003, I think that your only chance is the name, and whether you use the UniqueName or the Name to search the “Solution Items” string, take into account that can be localized (not tested) if you deploy it to international non-English IDEs.