OK, so this is a total rant:
The last time I called, I mentioned that I never, in all my years of going to WDW, have ever been given a code. On looking it up, the CM tells me "oh, no wonder--you have a 'do not contact with offers' note on your account." Are you kidding me?! I go multiple times a year, and I have a sign next to my name saying "under NO ACCOUNT give me any discounts?!" Is it sitting next to the sign that says "in fact, this customer would prefer to be charged double?"
He said he would change it, but since I have tried unsuccessfully to get them to change the incorrect address on my account to my real address at least 3 times, I'm not that hopeful.
So to answer the question, yes, apparently they do email or mail codes out at random, depending on whether they feel they need more people in the rooms, and the public codes are released at semi-regular intervals and usually announced on various message boards. Another way to find out about the public ones is to sign up for the mailing list for some of the Disney-specialized
Travel Agency websites, because they usually email everyone with the codes to encourage people to book.