Here's another cool library for AIR applications. It works for both Windows and Mac environments, and uses the NativeProcess access of AIR version 2 to tap into the built-in voice synthesis functionality of the operating system. It's still in development, is more developed for Mac than for Windows (at the moment) but it's looking pretty good already.
Should be a good one to have a play with soon!
Read the authors' blog post here: http://blog.technolog33k.fr/adobe-air/20-12-2009-voice-synthesis-in-adobe-air-with-the-nativeprocess-api/
Access the source code for the library from the Google code repository here: http://code.google.com/p/nativespeech/