As a Paleo-Eater, the first "fast-food" option that probably comes to mind is Chipotle. With their "food with integrity" slogan, this fast-casual restaurant offers a decent selection of quality veggies and meat.
My personal favorite is the salad bowl.
Lettuce, 1/2 Chicken, 1/2 Barbacoa, Guacamole, Fajitas, Tomato Salsa, and Red Tomatillo Salsa.
It's tasty, fast, cheap, convenient and (most of all) Paleo-Friendly.
Owner, Steve Ells almost gets it right-- That is, except for the the soybean oil that his restaurant chain uses. It isn't exactly an optimal choice of fats (ideally, we'd want coconut oil or avocado oil-- even olive oil), but it probably won't hurt in moderation.