It's a big problem, at least for our application (chattybar.com - a chat plugin). We have 2 copies of every chat message sent, one in the DB (for persistency) and one in Redis for quick retrieval. Keeping them synced has a lot of edge cases, especially since multithreading means that adding things to the DB and then to Redis doesn't necessarily guarantee they'll end up in the same order.
If all you're doing is keeping a copy of objects in a memory store for quick retrieval you might want to implement a read-through caching pattern using Memcached (or Redis but I'd turn any persistence off). Most chat clients don't allow for editing of posts, so you could just use Redis as your persistent storage using lists and RPUSH.
