This mode of doing things is, for the most part, already available. There are apps that work around the same principle as CalorieKing, where common restaurants and foodstuffs are saved in a database with their respective nutrition details. This tends to be very convenient when eating at popular restaurants, or eating commonly purchased grocery items.
The GPS element is an interesting twist on the idea though. How cool would it be if you walked into a Qdoba (just as an example), and your phone vibrated and asked you what you were ordering?
Yeah, I've seen that kind of wiki-like functionality in the Livestrong Nutrition app, and let's face the facts: preparation styles vary wildly among different restaurants. If I type "slice of pizza" into that nutrition app, I get "large slice," "cheese slice," "small slice," and several tens of variations. It's okay, but not ideal. If I go to the local pizza joint, I want to know fairly certainly what I'm getting in terms of calories if I order one of their specialty pizzas. That's where I think a database would be useful.
As for the GPS, that would be very cool.
As has been said before, the biggest obstacle to food apps is mobilizing a sales force to convince all restaurants to embrace a product. And because many restaurants don't really have (or need) IT capabilities, it's actually costly for them to implement computers and train staff to learn the particular system. That's why OpenTable and others are having problems with adoption.
The GPS element is an interesting twist on the idea though. How cool would it be if you walked into a Qdoba (just as an example), and your phone vibrated and asked you what you were ordering?