Created by the same dev that brought us the .FBX exporter:
http://peach.blender.org/index.php/trees/
Forum thread:
looks great. Have you had any opportunity to try low poly trees with it? GuessI’m gonna have to go through the old download Blender ritual again 
Cheers
Boxy
Unfortunately, this new script is only available in the current CVS build, and AFAIK there isn’t an OS X build available yet.