It connotes a certain sleaziness, because, as we all know, actually coding in CSS is yucky.
I spend 90% of my time doing skinjobs, hacking on CSS and messing with rollover graphics and flash and crap. I'd rather do the organization and planning parts, myself.
Additionally, used in the television series, Battlestar Galactica. Some resistance fighters on Caprica and members of the Colonial Fleet refer to the humanoid Cylons as "skin jobs," an homage to Blade Runner.
Battlestar Galactica: He poked a skin job... he racked up a six, that's a given... you don't have to get on your back for him, but...?