It’s a little counterintuitive, but one of the most important frontiers in web development is getting offline — taking applications that have been built online and allowing them to run even when people aren’t connected to the Internet. Adobe’s online-offline platform AIR launched a month ago, while Google offers its browser extension Google Gears, and recently revealed that through Gears, you’ll be able to use Google Docs offline. Now a San Mateo, Calif. startup called Etelos is throwing its hat in the ring with Apps on a Plane, which is launching in invite-only testing mode today.

The concept is pretty clear from the name. Etelos distributes business applications through the Etelos Marketplace, and now some of those applications will be able to run anywhere, even if you can’t connect to the Internet — let’s say, to use the obvious example, you’re on a plane. The startup’s approach seems rather different from Adobe and Google, which offer platforms for building hybrid applications. So if you’re a developer and you want your application to run on AIR or Gears, you need to either build a new app or substantially re-engineer your old one. With Etelos, on the other hand, developers can just input the basic rules of how their app should handle online and offline interaction, and AOP takes care of the rest, says founder and chief technology officer Danny Kolke.

Users, meanwhile, should barely notice a difference between the online and offline versions of the applications, Kolke says. Whenever they’re online, the data on their desktop will synchronize with the most recent data on the web. If they access the application offline, users just work with the most recently synced data, and everything is reintegrated once they log onto the web again. AOP also allows companies to create hierarchies where different users can sync (and therefore, access) different sets of data.

It’s obvious why the Etelos’ “let us handle the details” approach would appeal to a lot of developers — so obvious, in fact, that it’s a bit surprising to see a startup, rather than Google or Adobe, breaking ground in this area. But Kolke says being a relatively small company actually helped, because it forced the team to stay focused, rather than constantly arguing about the best solution. Apps on a Plane has been in development for between 18 and 24 months, Kolke estimtes, making it the longest proect in Etelos’ nine-year history. (The company offers a number of business tools, including its marketplace and customer relationship management for Google Apps.)

I haven’t had a chance to see Apps on a Plane in action, but it sounds like those months have been well spent. The concept is really exciting — let’s hope that AOP doesn’t blow the execution as badly as the similarly-titled film starring Samuel L. Jackson.

Update: Hot on the heels of this announcement, Etelos revealed today (April 23) that it’s done a reverse merger to become part of Tripath Technology, transforming the startup into a public company. The Etelos executive team says it will remain in charge and has applied for a new stock symbol. Each share of Etelos capital stock will be converted into Tripath stock at a three-to-one ratio.

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