e4Graph is an interest project, one I am keeping an eye on as the library matures.  e4Graph is a persistent graph storage library implemented on top of MetaKit.  Here are some relevant words to remember e4Graph by:

reliable, efficient, portable, persistent, storage, graph, C++

While MetaKit is mature, e4Graph is still very much a work in progress.  Still, I think the library will become more popular when we start to notice the limits of native XML databases.

Meanwhile, not having a .NET binding for Berkeley DB XML is frustrating.  Currently, I am using the C++ API in an ISAPI extension that handles access to DB XML.