The NY Times just had an item on this question:

Milk as Menace
By C. Claiborne Ray
June 20, 2011

Q. Some singers swear that drinking milk creates phlegm. True? And do dairy products make a cough worse?

A. There is no evidence that dairy products, especially milk, increase phlegm or the viscosity of phlegm, said Dr. Lucian Sulica, director of voice disorders and laryngology at New York-Presbyterian Hospital/Weill Cornell Medical Center.

"The question has been formally investigated in studies, which demonstrated no increase in mucus production," Dr. Sulica said, "although subjects who believed in the phenomenon reported that they did feel more mucus" when they ate dairy products.

One of the studies went so far as to expose subjects to a rhinovirus and then, when they developed head colds and blew their noses, to collect and weigh their used tissues.

Certain dairy products may cause the reflux of acid from the stomach, which may cause vocal difficulties, and certain people may have allergies to dairy products, Dr. Sulica said, but there is no evidence of a direct, broad effect of dairy on mucus production.

The corollary to this finding is that dairy products have no effect on cough, he said.

"That said, many vocalists do indeed believe in the adverse effects of dairy products," Dr. Sulica said. "Clinically, that creates the setup to the classic vaudeville routine:

" 'Doctor, it hurts when I do this.'

" 'Well, then, don't do that!' "