I may be missing something here.  The row does get validated as far as
routine non-key fields are concerned.  No problem there.  

However through trial and error I did find a solution to my problem, in the
DataError handler, which essentially consists of the thing ignoring those
IndexOutOfRangeExceptions I mentioned.  Here's the code.  Does this make
sense?

Private Sub dgFlowComp_DataError(ByVal sender As Object, ByVal e As
System.Windows.Forms.DataGridViewDataErrorEventArgs) Handles
dgFlowComp.DataError
       Dim nReturnCode As Integer = MsgBoxResult.Yes
       Dim nDialogStyle As Integer = CType(MsgBoxStyle.YesNo, Integer) +
CType(MsgBoxStyle.Exclamation, Integer)

       If TypeOf (e.Exception) Is ConstraintException Then
           nReturnCode = MsgBox("Error editing grid, error = " +
e.Exception.Message + ".  Do you wish to discard changes?",
CType(nDialogStyle, Microsoft.VisualBasic.MsgBoxStyle), "Warning")
       ElseIf Not TypeOf (e.Exception) Is IndexOutOfRangeException Then
           MsgBox("Error editing grid, error = " + e.Exception.Message)
       End If

       If nReturnCode = MsgBoxResult.Yes Then
           dgFlowComp.CancelEdit()
       End If

       e.Cancel = True

   End Sub