[quetwo] aka Nicholas Kwiatkowski developed a native interface to receive serial data in Flash.

[…]  The AIR Native Extension (ANE) is a C based .DLL / .framework for the Windows/Mac platforms that allows AIR to essentially open a COM port.  I wrote it in a way that is supposed to emulate the functions of the flash.net.Socket library that is included in the AIR runtime.  I’ve posted the entire project, including the source code and final binaries on Google Code at http://code.google.com/p/as3-arduino-connector/

via [quetwo], [GoogleCode]