The sub shops mentioned earlier are a good way to go. Hardees is offering a low carb, especially breakfast item in some locations. Taco Bell has a power menu bowl instead of a burrito, they have a breakfast one too. You can customize it to leave off the beans, rice, potatoes, etc and add extra meat...but it will cost you, they hit with a charge every time you say, change cheddar to three cheese blend for instance.
I'm finding the fast casual pseudo-Mexican places' salads offerings are pretty good. Just like Taco Bell you can opt for the salad instead of a wrap or taco shell then you just DONT have them put beans, rice, etc on and you can add extra meat for a reasonable price. One we found recently is Qdoba, the ingredients were varied and very good and they dont nickle and dime you on every ingredient...like guac is free for one measured portion for instance, more is extra but what they gave me was fine for free.
This guy is doing a whole series of fast food low carbing:
He also has recipes. But he helps cut through the menus at fast food places and breaks down your limited options and brings to attention items that may seem like they would be low carb but in fact are high carb. Subway is a funny one like that, they have a spinach wrap that looks like it would be a healthy option, but if you look on their menu, you'd be better off having a sub roll for the carb count!
I cant for the life of me figure out why the Mexicanish/burrito places arent offering low carb tortillas...even our local walmart has offerings of reasonably priced low carb tortillas, and now they are carrying Josephs low carb pitas for like $2.25 for a pack of 6...and they are good!
I've seriously considered carrying my own tortilla or pitas in a restaurant and eating my low carb order with them...maybe they'll get smart and get the hint! Problem is you have to anticipate that and have them with you, but if you know you are traveling, if you are like me I always have a cooler with me. Tortillas and pitas keep for a while unrefrigerated and keep fine after opening in a cooler or fridge (like in a hotel room fridge!).
Now, when will someone make LC hamburger buns and sub rolls that arent highway robbery price-wise?!