Sarah Perez wrote an interesting post on ReadWriteWeb today in which she asked “why doesn’t Southwest Airlines have an iPhone app?

In the comments were some great suggestions, along with the usual group of “iPhone and apple are evil” and “just do a mobile web browser” points of view.

I commented that several of Mashery’s customers, including the likes of Netflix and WhitePages.com, got their iphone apps (and in Netflix’s case, a blackberry app and inclusion on the windows mobile deck) by having an API.

That an API is the FasTrack (EZPass to those of you on the East coast) lane to an iPhone, Blackberry or mobile app shouldn’t come as a huge surprise, for two reasons:

1. For the geeks among us, a typical web app is three layers – the data layer (what we know), the logic layer (how we manipulate the data), and the presentation layer (the UI that is rendered on a web browser. An API bypasses the presentation layer and gives direct access to the logic and data layers. Since a mobile app is simply a new presentation layer – a new UI that makes different use of the data and logic layers – it follows that the easiest way to get a new presentation layer to work is to start with an API, and not to design all three layers anew; and

2. Southwest’s core competency is being an airline. They have also proven that they are quite good at creating a user interface and web experience that makes it simple for non-geeks to buy airline tickets. But this does not mean that they have a similar skillset on making great mobile apps. Sure, as one commenter on the RWW post said, a mobile app can just be something that runs on the browser. But that is not necessarily the best experience possible. Using location info, it can default to listing flights from the closest airport, for example. A great mobile app designer will understand how to make this a great experience, and having a bunch of these designers all working on apps will allow the best user experience to get the highest ratings and most traction.

Companies often have very passionate customers. Some of those customers also are developers, and they want to build a great app – and will do so for free.

As it turns out, the open API is the quickest path to a mobile app. Who knew?