BinarySludge.com: Redundant and Fault Tolerant PHP Session Storage
If a PHP application has deeply embedded usage of the $_SESSION superglobal, removing state is difficult. Instead removing the dependency between a user's session data and the single server it's stored on achieves the same fault tolerance.
They focus on a REST-ful approach to session handling, that it should be "kept entirely on the client" so there's no issue if something happens to the primary session data source. They point out that, while the technologies that can be used to replace it are similar, there's still some issues around using things like memsached, Sharedance, Hazelcast or MySQL to store session details.