There's countless websites that relate seb. derm. to yeast.
Here is a quote from one of them:
The main cause of seborrheic dermatitis is now believed to be overgrowth of normal skin yeast called M. Ovale. This conclusion is based upon the facts that the organism is found on people with the condition, can be grown outside of the body, and when placed on unaffected skin can cause the condition to develop. The way in which this yeast causes redness and scaling is not entirely known.
Here are two more:
http://dermatology.about.com/cs/seborrhea/a/sebderm.htm
http://www.sfsu.edu/~shs/skinclinic/seborrheic.htm
I have a book called "The Yeast Connection" which goes into detail about seb. derm's correllation with yeast overproduction, and I even asked my doctor about it about two months ago.
That's not to say that all seb. derm can be treated with oral yeast-fighting medications, at least not natural ones. I, for one, had an overproduction of yeast that manifested itself on my tongue and scalp, my doctor confirmed this in a lab test. I've since been taking garlic, capryllic acid, pau d'arco, grapefruit seed extract, and small doses of tea trea oil (60mg) daily and my problem has been greatly improved -- in fact, at least for the time being, I have no build up whatsoever on my scalp and/or tongue.
It should be mentioned that I also use nizoral, selsun blue, and head n' shoulders in rotation. However, i've used these products for years (literally) and they've done little to help my seb. derm, at least by themselves.