I just uploaded to svn.zope.org a new package called z3c.sharding.  It is a BTree container that spreads its contents among multiple databases in an automatically balanced way.  It does all of its work with pure and simple ZODB mechanisms.  It should be useful for solving certain kinds of scaling problems.  Enjoy!  There are no releases yet, but go ahead and try it out; if it solves a real problem in its current state, I’ll make a release.