I used to pronounce it Droh. For the longest time. Recently I converted to the drow (cow) version. And here's the further proff right out of the Forgotten Realms, Races of Faerun book: "They became known as the dhaeraow (the elven word for traitor), and over the centuries this word has since given them the name by which they are known: drow"
Dhaeraow: with the a in there gives the cow pronunciation. That's my thought on the subject anyway.
