There is also a new backend for elephant based on postmodern(cl-postgres) called ele-postmodern. It is thread safe and process safe. It is in alpha state but is already used by two production web sites. It is not, like perec, an object relational mapper, but merely uses postgresql as a low level storage mechanism.
Even though I make this ad about the ele-postmodern backend, I believe that there is much to do in this area. We have plans to make a new even better persistence solution, where better means safe, highly scalable for lots of processes, good performance, nice high level features. Maybe you are interested in coordinating efforts?
/Henrik Hjelte