The Amazon Kindle 2 has a feature that will convert any book you buy on the Kindle into an audio book, they call this the text-to-speech feature. To turn on text-to-speech while you are reading a book, press the Menu button and use the 5-way controller to select the "Start Text-to-Speech" option.

Once text-to-speech is running, you can press the Text key (Aa) to adjust the controls. You can change the speed at which the reading is done, and you can toggle the voice between a male and female voice.