Thanks Mattias,

I ended up writing my own routine, which won't require interop. It's not
tested through all the way, but it works for what I need at this point:

/// <summary>
/// Returns a relative path string from a full path.
/// </summary>
/// <param name="FullPath">The path to convert. Can be either a file or a
directory</param>
/// <param name="BasePath">The base path to truncate to and replace</param>
/// <returns>
/// Lower case string of the relative path. If path is a directory it's
returned without a backslash at the end.
///
/// Examples of returned values:
///  .\test.txt, ..\test.txt, ..\..\..\test.txt, ., ..
/// </returns>
public static string GetRelativePath(string FullPath, string BasePath )
{
  // *** Start by normalizing paths
  FullPath = FullPath.ToLower();
  BasePath = BasePath.ToLower();

  if ( BasePath.EndsWith("\\") )
     BasePath = BasePath.Substring(0,BasePath.Length-1);
  if ( FullPath.EndsWith("\\") )
     FullPath = FullPath.Substring(0,FullPath.Length-1);

  // *** First check for full path
  if ( FullPath.IndexOf(BasePath) > -1)
     return  FullPath.Replace(BasePath,".");

  // *** Now parse backwards
  string BackDirs = "";
  string PartialPath = BasePath;
  int Index = PartialPath.LastIndexOf("\\");
  while (Index > 0)
  {
     // *** Strip path step string to last backslash
     PartialPath = PartialPath.Substring(0,Index );

     // *** Add another step backwards to our pass replacement
     BackDirs = BackDirs + "..\\" ;

     // *** Check for a matching path
     if ( FullPath.IndexOf(PartialPath) > -1 )
     {
        if ( FullPath == PartialPath )
           // *** We're dealing with a full Directory match and need to
replace it all
           return
FullPath.Replace(PartialPath,BackDirs.Substring(0,BackDirs.Length-1) );
        else
           // *** We're dealing with a file or a start path
           return FullPath.Replace(PartialPath+ (FullPath == PartialPath ?
"" : "\\"),BackDirs);
     }
     Index = PartialPath.LastIndexOf("\\",PartialPath.Length-1);
  }

  return FullPath;
}