Using a combo of the append-only file[1] and a redis slave[2] you'll get about the best durability one can ask for in today's world (assuming you're aware of the gotchas with regard to expiring keys — an edge case).

[1] http://code.google.com/p/redis/wiki/AppendOnlyFileHowto [2] http://code.google.com/p/redis/wiki/ReplicationHowto