Jason Horn's bio on his Chow.com stories is one place that mentions it, but nearly everyone who leaves the Bay Area or California laments the lack of Mission-style burritos. Oddly enough, the first time I heard about Mission-style burritos was when I lived in Portland and the nearest place to eat a cheap lunch indoors (a key thing to know when you live in a rainy city) was Taco del Mar.
I thought one of the funny things about this post on Emdashes was that tacos were cited as a reason to visit California back in the 1970s. Is that our culinary heritage?
So what is a Mission-style burrito? I think, for most of the world, it's a non-Taco Bell burrito wrapped in tin foil. This Chowhound thread seems to say that the difference is rice, which my friend Prentice would take issue with. She points out -- rightly, I think -- that the rice is just really filler. Yes, it does absorb the liquid of the salsa, tomatoes, etc., but the quantity of rice in most burritos leaves me feeling that, even for a $5 burrito, I'm paying for more than I'm getting.
Of course, Wikipedia weighs in on the subject. My favorite section is the culture and politics. Only in San Francisco would a burrito become politicized.