Apple has created lots of new speech synthesizer voices for Lion. It's easy to download them, but the fact that they exist isn't very-well advertised in System Preferences (or otherwise in the OS). Here's how to get them.
- Open System Preferences » Speech » Text to Speech.
- Click on the System Voice: menu and select Customize.
- You're presented with a huge list of voices to choose from.
- Click on a voice and press Play to preview it.
- Check the boxes to the left of the ones you like. Then click OK.
Software Update will open and download your voices. Be warned, some of the files are huge.