The feature was posted on Reddit so we don’t have many details beyond that, but it looks pretty cool.
It looks as if all you have to do is select a block of text, alt-click it and choose the ‘Add to iTunes as a Spoken Track’ option from the menu. You’ll then be presented with a dialog box that allows you to choose a name for the track. Once you’ve hit continue, the text track will be added to iTunes and you can play it back.
Update: As Twitter user @gabek pointed out to us, this is actually already built in to OS X. It’s provided as an option under System Preferences>Keyboard>Services. You have to tick the box to turn it on. So, still a very cool feature of OS X, just not new to Lion.