Add a change history group to your form to track changes. This XML Template Part is a control. Download it and add it to your Designer using the Add Custom Controls. After you have added it to the custom controls section, just drag and drop it into your view. Only works with 2010 and may require some extra work to enable in browser (since it uses filtering to show only the last node in the group).
Great example of how to use complex xpath to extract and set values from a repeating table.