Corey Ballou has posted a MongoDB-based session handler to his site with a new update back on July 1st.

MongoSession is a PHP implementation of a MongoDB session wrapper. This class was built as a drop-in for easily switching to handling sessions using Mongo. It's a great replacement to memcache(d) for VPS servers where you risk memory being reshuffled in the pool and taking performance hits.

You can grab the latest version of the source from the github repository for the project or just copy and paste the source directly from this page on his site. A usage example is also included.