Never going to happen. The books are too niche to carry in the book section for most stores, and there is no place in toy's/games section for books. Minis take up too much shelf space and sell too little for the overhead to carry them. Cards take up less space and have higher profits so are easy to carry a selection in some hidden place.
With the recent changes to Wal*mart store designs thee is going to be less space for things as the shelves themselves have been made shorter so they will be picking and choosing more of what they carry meaning less chance for 4e or DDM in them. You would have to live in a heavy gaming area in order to see these things, and most stores aren't in those areas for the Wal*mart warehouses to keep them in stock.
I don't think I'd say never. With the books, probably not, but that doesn't mean never. More than likely you'll probably only see them in bookstores (Books-a-Million, etc.), but again, never say never. As to DDM, they are already there. I've seen both D&D mini booster sets and a large amount of Star Wars minis (not the toys, actual Star Wars RPG mini booster sets). Perhaps you haven't seen them in the Wal*Marts you've been to, but I have seen them in ones I've frequented.
I also wouldn't say that it depends on a heavy gaming presence. I've seen this in four different places; Grand Rapids, Michigan - Valdosta, Georgia - Palm Beach, Florida - Orlando, Florida. Grand Rapids and Orlando could be considered your standard big cities, so it's reasonable they would carry them. However, Palm Beach has only one game/comic store for an area with about a 50 mile diameter - not exactly a gamer heavy area. Valdosta is even less gamer heavy. They lost there only RPG store to a CCG and Comics only store near the local college. Eventually an RPG store did surface to fill the niche but was located 10 miles north of the city outside a local military base, where there is a niche group. For a while there was no RPG store in Valdosta, other than buying books at the local Books-a-Million. Definitely not what I would call gamer heavy, and I've seen minis in every single one of those Wal*Marts.
I don't know about recent changes to Wal*Mart store designs (other than getting bigger), but I saw Star Wars minis just this week at a local Wal*Mart. I'm not sure where you get your information about what Wal*Mart is able to keep in their warehouses, but it seems that the fact I've actually seen them proves this wrong.
I've also seen them fairly regularly in Target.
Edit: The Little Raven posted while I was typing so I didn't see where he said he'd actually bought the books in a Wal*Mart. Personally I've never seen the books in a Wal*Mart, but I haven't actually looked for them either. I usually don't look at the books in Wal*Mart, I'll go to one of the book chains instead.