Note that it's in the process of being released: it's as yet unusable by outsiders. Honestly, I did not expect this to make Hacker News so soon. :(

I have not used Neo4J first hand. It has really cool features, but it is not a distributed database and has expensive memory usage. FlockDB is distributed, uses little memory, and has a very limited feature-set that is highly highly optimized for OLTP. It's not really an apples/apples comparison. Theoretically, Neo4J could be used as a back-end data-store in FlockDB.