See the question and my original answer on StackOverflow

Here is some sample code that demonstrate how to access the selection in the property grid. Note there can be multiple objects selected, not only one:

IVsMonitorSelection selection = (IVsMonitorSelection)yourSite.GetService(typeof(SVsShellMonitorSelection)); // or yourPackage.GetGlobalService
IVsMultiItemSelect ms;
IntPtr h;
IntPtr pp;
uint itemid;

selection.GetCurrentSelection(out h, out itemid, out ms, out pp);
if (pp != IntPtr.Zero)
{
    try
    {
        ISelectionContainer container = Marshal.GetObjectForIUnknown(pp) as ISelectionContainer;
        if (container != null)
        {
            uint count;
            container.CountObjects((uint)Microsoft.VisualStudio.Shell.Interop.Constants.GETOBJS_SELECTED, out count);
            if (count == 1)
            {
                object[] objs = new object[1];
                container.GetObjects((uint)Microsoft.VisualStudio.Shell.Interop.Constants.GETOBJS_SELECTED, 1, objs);
                object selection = objs[0]; // selection is here
            }
        }
    }
    finally
    {
        Marshal.Release(pp);
    }
}