Interesting work from Newcastle suggests, as sqad says, that some cases of type 2 diabetes can be reversed with a low calorie diet - it will be interesting to see the results of a more extensive trial, as the original study involved less than 12 people, so early days yet.
Sqad, my understanding of it was that you didn't have to keep on a very low calorie diet for ever, but it was enough to keep your weight down below a certain level.
http://www.ncl.ac.uk/magres/research/diabetes/reversal.htm
NB This kind of dieting should only be carried out under strict medical supervision.