The Effects of diet on Acne breakouts has been and still is a much debated topic.  A search on pubmed.gov reveals well over 100 different studies involving the effects of diet on Acne.

One study in particular studied 4,273 boys over the course of 1996-1998.  The findings showed a correspondence between milk and acne.  Surprisingly the greatest correlation was found with skim milk.  One key weakness in this study; however, was the fact that it was conducted through questionnaire so there is room for variance. (Dermatol Online J. 2006 May 30;12(4):1.)

Another similar study was conducted on adolescent girls and revealed similar results. (J Am Acad Dermatol. 2008 May;58(5):787-93. Epub 2008 Jan 14.)

Anyways current research is showing more and more a startling correlation between milk drinkers and acne sufferers.  However, studies thus far have been self evaluated and required people to remember things; therefore, there is significant room for error.  So until further research either confirms or denies it would be a good idea to conduct your own study and see if reducing your milk/dairy intake improves your skin.

Another interesting article was about a study done on bread intake and acne.  The study speculates that due to the ease with which our bodies digest many processed breads, this causes an Insulin and Insulin-like Growth Factor spike within the blood.  This increase in insulin has been shown to negatively impact acne and acne outbreaks.

http://news.bbc.co.uk/2/hi/health/2542801.stm)

Whatever the truth about diet and acne is, the best advice is to just pay attention to what you eat and how your skin reacts.  In fact, Keeping a journal and recording what you eat and how your skin reacts is the best way to understand how diet affects you and your skin.