The D&D ruleset is heavily dependant on magical items to keep the classes in balance. A low-magic campaign using stock D&D rules would have severe balance and gameplay problems. Hence, a low-magic setting would have to signifigantly alter the core rules. Because of that, I don't think its reasonable for WotC to release a D&D setting that is lower magic than the rules implicitly require.
Its probably better for people who want a low-magic campaign to play some other game system. If people still want to play d20, d20 Modern works perfectly well for a low-magic fantasy game with only a few minor tweaks. I've done it, and it works well, and it retains a signifigant amount of D&D feel, as it uses the same monsters, magic items, and spells, for the most part.
Would I buy a low-magic setting if it came out? No. I don't buy campaign setting books. I prefer to just make stuff up.