Borrar coma al final de cadena.

[colombianDreams]

Lo solucioné con esta linea:

For Each dr As DataGridViewRow In dgv57.Rows
For Each dl As DataGridViewColumn In dgv57.Columns
If dr.Cells(dl.Name).Value IsNot Nothing And dl.HeaderText <> "Detalle" Then
strExport += dr.Cells(dl.Name).Value.ToString() + ","
End If
Next
'con esta linea
strExport = strExport.Substring(0, strExport.Length - 1)
strExport += Environment.NewLine.ToString()
Next

gracias igualmente.