Found an interesting quote from from EdREN, the website of the Renal Unit of the Royal Infirmary of Edinburgh:
"For some reason, once kidneys have been damaged, they may continue to get worse over months and years, even long after the disease that caused the damage has gone. This is more likely if you have high blood pressure, and if there is much protein leaking into the urine."

http://renux.dmed.ed.ac.uk/EdREN/EdRenINFObits/CRFLong.html

This is a bit scary as it suggests that the kidneys usually continue to degrade even in the absence of anything attacking them. I hadn't considered this previously and had imagined that if I managed to convince my immune system to stop doing whatever it's doing to my kidneys that things would be okay. Unfortunately I have had both high blood pressure (controllable) and significant protein in my urine (1881mg in 24 hours in a test on the 06-Mar-09). I don't know what can be done to stop protein leaking into the urine. Possibly eating less protein (which is probably why the nefrologist has put me on a low protein diet)

The website also shows a graph showing a patient's kidney function degradation over time, taking only six years from treatment (which led to initial improvement) to dialysis. Scary, scary stuff!