Actually, the flash platform is largely open source. The AIR framework (along with the rest of Flex) is open source. See http://opensource.adobe.com/wiki/display/flexsdk/Get+Source+...

The AIR compiler is also open source. The only thing that isn't open source in the stack is the flash player runtime, which executes the resulting SWF file. However, the SWF file format is open.

If the VM - the most critical part of the platform - is proprietary, then the platform isn't largely Open Source.

http://titaniumapp.com/ if you want an open source AIR equivalent. It's a work in progress but seems promising.