This is kinda what mongodb[0] does now and is a question of durability[1]. Current rdbms systems that lay claim to consistency must flush to disk to ensure ACID[2] compliance. Redis does not claim such compliance and I am of the opinion that it should not.

[0]http://www.mongodb.org/

[1]http://en.wikipedia.org/wiki/Durability_(database_systems)

[2]http://en.wikipedia.org/wiki/ACID