Won't it be in UTF-8 format by default because your MXML application starts with:
<?xml version="1.0" encoding="utf-8"?>
Okay, got it.
You can use the IDataOutput interface; it has got a writeUTF() method that takes in a string as argument. See this: http://livedocs.adobe.com/flex/3/langref/flash/utils/IDataOutput.html#includeExamplesSumma ry