I'd say it's probably a nutrition and sunlight thing. Pumpkin plants are vines and they get rather large. Might steal sunlight from your potatoes.

I stumbled upon this advice today, but can't find the reason WHY they should be kept apart. Mine ARE next to each other. My spuds are nearly ready to harvest.. please god, tell me they won't be orange! haha..

Seriously though, why should they be kept apart, and what happens when they're not?