Adobe’s AIR for Linux, the runtime engine that supports rich Internet apps (RIAs), has caught up with its Windows and Mac siblings.

The company has released AIR 1.5 for Linux, the first time the Linux desktop variant has made it as a production-grade, Adobe-supported release.

And now also for the first time AIR for Windows, Mac and Linux all have the same feature. For a while there the Linux 1.1 beta had widgetry that wasn’t in the others.

Adobe has been less pressed to come up with the Linux cut because Linux is pretty scarce on the corporate desktop; it quotes Gartner’s 1.8% number. With the new arrival, Adobe suggests that may change if only because Linux can now poach other operating systems’ RIAs.

Developers and designers can now target the three platforms with the exact same code without making any modifications – well, at least 90% of the time or slightly better, according to Adobe’s tests.

The Linux stuff supports Fedora Core 8, Ubuntu 7.10 or higher, and openSUSE 10.3. Adobe couldn’t explain why it was supporting test bed, community Linuxes like Fedora and openSUSE and not the finished goods.

It’s also working with Intel-supported Moblin Linux crew to get AIR on Atom-based netbooks and nettops and has cozied up with Android and Nokia for phones. (It’s working on Flash for iPhone but has to overcome Apple’s distaste for the widgetry.) It also expects to support China’s Red Flag Linux at some point.

AIR, of course, is a key component of Adobe’s pervasive Flash platform and enables web developers to use HTML, JavaScript, ActionScript and the open source Flex framework to deliver web apps outside the browser. It also accelerates performance with the SquirrelFish WebKit HTML JavaScript interpreter.

And the 1.5 rev includes a new encrypted database that meets enterprise security compliance requirements.

AIR for Linux is available in the same 15 languages as Flash Player 10.

AIR is free so it won’t help Adobe with its immediate problem – flat sales. The company said the other day that its earnings in the November quarter, a nasty period of time for practically everybody, were up 11% to $245.9, 46 cents a share, but sales, well, sales were only up $4 million year-over-year to $915.3 million.

Adobe’s expecting revenues to drop this quarter 5%-10% and has already cut 600 jobs, reportedly in one afternoon.

It’s also facing a determined Microsoft wielding its AIR-competitive Silverlight technology and willing to make what deals are necessary to win despite the fact that the Flash Player is on an estimated 98% of the PCs out there and Adobe’s ahead on the feature side while Silverlight’s only on 25%.