In addition to high-computing speed, Curl has (according to my own speed tests) very high-performance graphics, at least on the Windows platform when using DirectX. It also has demonstrably strong security. (Curl has criticized the Adobe AIR security model as having a huge hole in its signing certificate scheme.)
The Curl language may be new to you, but it isn't hard to learn. In addition to providing copious documentation, Curl supplies many excellent samples with source code, a number of interactive tutorials on application-specific topics called Curl Cues, a Curl IDE Made Easy interactive training program and online courses on the Curl language, the Visual Layout Editor, and the Curl application development process.
I was initially baffled when I looked for the Nitro extensions in the current Curl IDE documentation. Then I looked at the source code for the three Nitro samples supplied on the Curl Web site, and that made it clear where to find the Nitro functions in the Curl libraries. One of the Nitro samples did not work for me, however, because of a problem with the Facebook API. The other two samples worked very well.
The Curl RTE (runtime environment) runs on Windows, Mac OS X, and four versions of Linux. The Curl IDE runs on Windows and Linux. The Curl development plug-ins for Eclipse work with Eclipse 3.3 and 3.4 on Windows and Linux platforms.
The base Curl IDE and runtime that include Nitro are free, as are deployment licenses for publicly available, free Web sites that use the base capabilities. A Curl Pro/Deployment license starts at $12,000; Curl has pricing models designed for enterprise, Internet, software-as-a-service and reseller business models.
Basically, Google Gears is a way to take Web applications offline in a browser. Curl Nitro and Adobe AIR can go one step further and dispense with the browser.