A new study to be published in March 2013 in the Journal of the Academy of Nutrition and Dietetics reviews all the relevant scientific literature from 1960-2012 and concludes that there is a clear link between 1) eating dairy and acne and 2) eating foods with high glycemic index/glycemic load and acne.
High glycemic foods are those that raise blood sugar quickly, and include sugary foods, juices, potatoes, white rice, and foods made with white flour including breads, pretzels, pastas and cereals.
Skim milk has a worse effect on acne than whole milk because there is no fat to slow down the absorption of the milk sugars.
The study authors conclude that nutritional therapy (changing your diet) ought to be used by dermatologists in conjunction with other treatments. I would even suggest, if you have acne, to clean up your diet first. You may not even need the dermatologist.