I am more of a Flex and AJAX developer than a Flash designer. I found the Air extensions to Flex fairly straightforward to learn and use from Flex Builder, although I would have been happier if the Air documentation were included in the Flex Builder help file instead of only being available online.
Air runtime, Air SDK, Flex 3 SDK and Aptana Studio are free. Pricing for Aptana Studio Pro is US$199, US$699 for Adobe Flash CS4 Professional, US$249 for Adobe Flex Builder 3 Standard, US$699 for Adobe Flex Builder 3 Professional and US$399 for Adobe Dreamweaver CS4.
Nitro is the cross-platform desktop extension to Curl. Currently in its second beta-test release, Nitro adds an applet installer, desktop controls and a client-side SQLite database to Curl's excellent RIA capabilities. Nitro applications take advantage of Curl's high-performance, skin-able user interface and built-in security sandbox. Even without Nitro, detached Curl applets can be used for desktop applications: The Curl IDE and help system are good examples of that.
Curl may not be as familiar to you as the competing products from Adobe (Air), Google (Gears) and Microsoft (Silverlight), but you shouldn't let that deter you from trying it. It has CPU-bound runtime performance roughly 10 times that of Adobe Air , according to a test of JPEG encoding conducted by Curl in May. (The test was pooh-poohed as "irrelevant" by Adobe , but not refuted.)
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.