There is ONE roleplay currently active; but it is a "social group" (requiring invitation). It is called "Knights and Ladies." The member who started it is called SusanII; if you can catch her on-forum, tell her you're interested. The setting is not IN Narnia, but it is in a similar kind of parallel dimension, with human inhabitants who originated in OUR world.
Something you can do right now to determine if you would like to join K-L, is to enter the Writing Club section and read a story called "Theocrites the Archer." I wrote that for the exact purpose of giving people a look at the roleplay-universe.