They just announced a new version of their PC app, and it's based on Adobe Air. From the press release:
Zinio, the global leader in digital publishing technology and services, today announced the launch of Zinio Reader 4 (zinio.com/reader4), built using Adobe AIR. In combination with Zinio's latest product launch, National Geographic celebrates World Water Day with a free download offer of its interactive April 2010 "Water" issue, using Zinio Reader 4 and "UNITY" platform.Zinio's newest digital reading application enables a seamless, online or offline reading experience across the Company's global offering of over 2,000 top consumer magazines. Zinio Reader 4 was built using Adobe's AIR framework, optimized to power a cloud-enabled library, specifically tailored for interactive magazine and book content experiences. This latest release from Zinio is another addition to its continued focus of offering ubiquitous access across PC, Mac, netbook, touch-screen and tablet devices, including the iPhone, iPod touch and the upcoming iPad from Apple. An extension of Zinio's pay-once-read-anywhere "UNITY" platform, Zinio Reader 4 is supported on Windows, Mac and Linux operating systems, in its initial beta release. This new application introduces enhanced features enabling consumers to access their favorite interactive magazines and books worldwide, online or offline, in a more optimized and enjoyable way.
The app can be downloaded now, and a special release of the latest issue of National Geographic is available for it as a free download. You can get that issue here, but you'll need to set up an account with Zinio first. Unfortunately, the app is broken at the moment. It can't access the Zinio website to download content, but this will probably be fixed fairly soon.
I opened the special issue in the browser based Zinio reader, and I'm pleasantly surprised. I was expecting something like a PDF viewer, with pages that were simple copies of the print edition. Instead, the content scaled to match the browser window, and there was a scroll bar for when there was more text on the page than could fit on a screen. This app was quite usable.