I need to develop a graph database in python (I would enjoy if anybody can join me in the development. I already have a bit of code, but I would gladly discuss about it).
Regarding Neo4j, did you notice the existing Python bindings? As for the disk storage, take a look at this thread on the mailing list.
For graphdbs in Python, the Hypergraph Database Management System project was recently started on SourceForge by Maurice Ling.