Tim cites this study: http://www.ajcn.org/content/74/1/96.full, which basically concludes that while milk products have a low glycemic index, they have a high insulinemic index, meaning they cause insulin concentrations to be higher in the blood.
Regarding the study cited by Andrew Ferk, I'm curious about the experiment design, especially the fact that the control group is being fed corn starch, and the experimental group is getting lactose.
It's of concern that the testing isolates lactose instead of feeding the rats milk products, which would include fats and proteins. It's also of concern that the protein:fats:carbs ratios are very low in protein, either 11 or 13 percent.
I think this all adds up to Tim's main point, which is this book is a record of a number of experiments he undertook on himself, and if you aren't approaching it the same way, you might not have good results.
So, maintain a steady diet, and try a month without milk, and a month with, keeping the other balances the same, and see how it affects things.