Much like Java, Flex compiles code into a bytecode representation for the Flash virtual machine (Flash VM). While opening VM code using a text editor, certain tools are able to retrieve significant portions of information for compiles Flex app. Much like obfuscators for JAVA, obfuscators for Flex/AS try to hide as much as possible in compiled code.

As always, you can’t obfuscate all. How much an obfuscator helps, depends on your needs and your project. Here is a (imcomplete) list of tools:

http://ambiera.com/irrfuscator/index.html [Free/BETA]
http://www.kindisoft.com/secureSWF/

Genable, another obfuscator, has been sold by the developer to another company, but you can still access a preview here:

http://web.archive.org/web/20040201231108/www.genable.com/aso/preview.html

Additionally, this video on InsideRIA presents NitroLM, a pro-class SWF encryptor. Take note, that NitroLM provides more than just encryption, since it is a complete license managemet solution.