Freeweed: As entheosfog noted, it is quite contested. That link to Juarez is not proven, but one of several theories. You'll note if you've had a Mexican burrito how much different it is from one you'd get at a Tex-Mex place. Generally better ingredients, and looks more like a Shawarma than the ball of grease that comes from Taco Bell. Even in the rest of North America there are many variations of the Burrito: Mission style, San Francisco style, Montreal style.
Same thing, but also very different.
Here is a particularly interesting little account of the Burrito:
http://mexicocooks.typepad.com/mexic...ose-of-yo.html