Redis is suitable for persistence, so where it would really be appropriate would be storing things like player profiles and other misc data that is not tied to a specific server. In fact, that's exactly what I'm using it for.
You might finally be able to break the model that current large games like WoW use - heavily sharded with all persistent data stored in Oracle. Redis should allow you to shard less and use SQL less, resulting in a much better experience, especially when parts of the virtual world get crowded.
I'm curious: what you think (or know?) they are doing that is causing the database to be a limiting factor?
If Redis can help them scale from the small shard to larger shards that support more players in an area, it might be interesting.