How to find a ProjectItem by the file name

May be a little late but use DTE2.Solution.FindProjectItem(fullPathofChangedFile);

I am using this user-friendly world of DTE as well, to create a Guidance. I did not find any better solution. Basically these are methods I am using:

Iterate projects:

public static ProjectItem FindSolutionItemByName(DTE dte, string name, bool recursive)
{
    ProjectItem projectItem = null;
    foreach (Project project in dte.Solution.Projects)
    {
        projectItem = FindProjectItemInProject(project, name, recursive);

        if (projectItem != null)
        {
            break;
        }
    }
    return projectItem;
}

Find in a single project:

public static ProjectItem FindProjectItemInProject(Project project, string name, bool recursive)
{
    ProjectItem projectItem = null;

    if (project.Kind != Constants.vsProjectKindSolutionItems)
    {
        if (project.ProjectItems != null && project.ProjectItems.Count > 0)
        {
            projectItem = DteHelper.FindItemByName(project.ProjectItems, name, recursive);
        }
    }
    else
    {
        // if solution folder, one of its ProjectItems might be a real project
        foreach (ProjectItem item in project.ProjectItems)
        {
            Project realProject = item.Object as Project;

            if (realProject != null)
            {
                projectItem = FindProjectItemInProject(realProject, name, recursive);

                if (projectItem != null)
                {
                    break;
                }
            }
        }
    }

    return projectItem;
}

The code I am using with more snippets could be found here, as a Guidance for new projects. Search for and take the source code..