Question, why not write and update data into memcache, and use the sql database as a back end backup? What I was thinking was, do everything in memcached, and backup data to sql on an hourly or semi hourly basis (one giant query is better than a million small ones) worst case scenario server crashes players lose an hour of their game, on the other the game will handle more players more smoothly and would save server costs. Premium related actions can be handled purely by sql so that money would not be in jeopardy.
I also think if the game is for example an RPG game, and you only keep active players data in memcached, you might be able to fit the whole active part of the database in your memory, if each player is worth 100 KB of data, and you have 1000 players active at one time... so um yeah, I hope to get a reply on this.