Sourdough Bread Starter with Sour Milk?
This doesn't sound like a very good idea. The problem of the milk spoiling and creating extremely bad flavors is a very real one.
It's also not a good idea from a baking chemistry standpoint. If you have a basic understanding of the chemistry of sourdough starters, they involve a lactobacillus (often lactobacillus sanfranciscensis, but sometimes another lactobacillus) and a yeast that coexist together. When the lactobacillus gets working in the sourdough, it gives off the same acid that's also in milk -- lactic acid -- as well as acetic acid, the same acid that's in vinegar. That's what puts the sour in sourdough.
So you see, there's no need to add milk (the only reason in my mind for doing so would be for the lactic acid) when the starter just by being itself creates its own lactic acid.
And the yucky aspects of the milk spoiling -- besides just tasting terrible --might disrupt the basic chemistry of the starter itself. By the way, the yeast that's in the sourdough starter, usually candida milleri or candida humilis, does most of the work to make the bread rise. The lactobacillus gives the sour flavor. For more info and lots of interesting tips on sourdough, please see this current Chowhound thread at
re: maria lorraine
And to round out Maria's excellent synopsis, a word about wild yeasts...
The sour in sour dough breads as Maria says comes from the production of lactic and acetic acids as the starter (or Barm as it is sometimes called) ferments. The acid would kill commercial yeasts but wild yeast has a high tollerance for acid and can survive to do its magic in the bread making process.