Voyage is another approach in the Smalltalk/Pharo world to persist object models. It was written by Esteban Lorenzano, I think mainly with the goal to persist data from Pier (the CMS system) images.
You will find his slides from the Pharo 2012 conference here.
Francois Stephany is a beta tester and is already blogging how to use Voyage in conjunction with MongoDB.
The code can be found in the Voyage project at smalltalkhub.