EVERYWHERE. JUST EVERYWHERE. This has probably been happening for ages, but it's like an epidemic, wiping out the dress and skirt. They've formed a symbiotic relationship with dresses and are then destroying them from the inside out with their horrible jersey clinginess.
I have heard of these new things called Jeggings, which despite the stupid name, can at least be worn on their own. But leggings...if the only reason for their existence is that they have to be worn with dresses, which are fine with out any legwear (and actually better), then they're just wrong.