How do you look at adding findByIds(array $ids) to EntityManager and UnitOfWork? This would allow fetching multiple entities from a database at one request and would be very useful for caching - there would be even some kind of IdentityMap kept in memcached or any other caching engine, that supports multiple id retrieval: i've been using such an architecture in multiple projects and it turned out to be very effective. There were two basic methods - findIdsByFilter(array $filter) and findEntitiesByIds(array $ids). The latter one had a caching proxy, replicating entities to a cache storage. If this idea proceeds - I'd be glad to cover it with more details.

This topic on StackOverflow could also help:
http://stackoverflow.com/questions/276709/design-pattern-for-memcached-data-caching