Riak does. You say, when writing, "please don't return until this data is replicated on 2 servers." And when reading, "please only return a successful read if this data is read from 2 servers."
So you have R = 2, W = 2, R+W = 4, and if your replication (N) val is 3, you're fine (you're always going to get consistency if R+W > N).
So you have R = 2, W = 2, R+W = 4, and if your replication (N) val is 3, you're fine (you're always going to get consistency if R+W > N).
Riak is cool.