The Linux version of Adobe AIR, a platform for developing and running Internet applications that run on your desktop, is now available in beta testing mode. That means Adobe is getting closer to a full launch of AIR on the Linux operating system, and also to its goal of making AIR a platform for truly ubiquitous “build once, run anywhere” applications.

AIR keeps pushing ahead and making itself more attractive to developers, while competing platforms like Sun’s JavaFX and Mozilla’s Prism are still in development or testing. I have to admit, after talking up the platform when it launched in March, I don’t actually use any AIR applications regularly, but I’ve been hearing about some good ones. For example, Twhirl, a desktop interface for micro-messaging service Twitter was recently acquired by video startup Seesmic.

AIR launched with PC and Mac compatibility, so Linux was the biggest remaining hole. An early, alpha test version became available in March, but the latest release is more complete. Adobe says the only missing pieces of functionality are badge installations (basically, buttons for installing an AIR app) and digital rights management. And in the case of the missing DRM — the technology that restricts the use of certain software and content for copyright protection — I’m not sure that’s a bad thing.