The procedure suggested by maclema will not really stop any attacker from obtaining the source - the "wrapper application" will need to be unencrypted so the attacker will be able to find out that you use AES (or any other algorithm) and he will obtain the decryption key in a similar way (because it needs to be in plaintext somewhere). Once he has this, he will be able to decrypt your SWF file easily.
The only reliable solution (well...) is some kind of obfuscator - we use Amayeta which works for Flex in the latest version - please see http://www.amayeta.com/software/swfencrypt/ .
