Yep, my bf and I noticed it when we were there in February. I had been several times in the past, but that's the first time I smelled the dirty diaper smell, and it was awful. We only passed through, we didn't stop to shop because it was that bad. It's a shame, too, because Mouse Gear is some of the best shopping in all of WDW.
The monorail smell totally reminds me of a horse stall. I'm not sure if it's the plastic seats or the flooring, but yeah, it reeks of horse.