I managed to translate Webbot's speech synthesis code and made a simple TTS (text to speech) library. But You can't connect just a speaker to a PWM pin, you need to add a RC filter and an amplifier. I used the schematic below:
Also, the code is big and the speech is retro robot like. You can't do anything else during the speech time, but you can do whatever you like before and after the speech is done.
Here is the lib: TTS.zip After download, create a TTS folder under Arduino-0018\libraries and place all files there. Then, from Arduino you can open the example provided.