This is worth a read Rob ->
http://www.astrosurf.com/~buil/eos40d/test.htm
He has tested 40D sensitivity and this page shows ISO 100 on the 40D gives the same results as ISO 400 on a 400D and 20D. I think, if you can read between the french, that he is saying they have cranked up the sensor gain in the 40D by a factor of 4.