No Disney does not allow stacking of codes. If they did I would use the free dining, plus my 40% PIN, plus my AP discount, etc and Disney would then be paying me to come down there. You must run the numbers and see which discount works best for your family. I currently have free dining for my Sept/Oct trip, but am hoping an AP discount comes out that beats it. I used a 40% off PIN for my Dec trip, but again am hoping either an AP discount or Visa free dining offer can beat it. I'll have to choose the one that saves me the most money. I can't combine them.
Now there was some bounceback offers last fall that gave free dining with 30% off certain resorts (Saratoga Springs and OKW). There was also some codes for the military that gave room discounts and discounted tickets. Those codes included both components though. They were not stacking two different codes.
PIN codes are sent out by Disney to individuals (either by e-mail or through the mail) and have different offers on them. They are only available to the individual named in e-mail and are not transferrable. And, as stated above, not stackable with other deals.
No one knows for sure how to get them. It seems like the more "Disney lists" (signing up on the website, ordering custom maps, signing up for a Disney Visa, etc) you can get your name on, the better your chances might be - but no one knows for sure.
I got my first one earlier this year - but it was for a cruise this summer that we could not use. I think it was kids cruise free or something.
kevgraub- thanks for the PIN info. Guess I'll just have to keep on planning to use my DVC points on my accommodations and use my cash for airfare & food!