There is a difference between multi-master and circular replication. To me, Multi-master is that I can write to both masters at the same time, which implies there is a way to resolve conflicts. Databases like Cassandra (timestamps) and Riak (vector clocks) have this, MySQL does not. If you write to the same record on both masters bad shit happens and its very hard to sort out.
You can write to both masters at the same time in a MySQL multi-master circular replication setup. It's done via auto_increment_increment and auto_increment_offset configuration settings in my.cnf - each server generates autoincrement keys that are unique to that server.
This still doesn't protect you from UPDATE statements. I suppose you could pull it off if you could be absolutely certain that you were only CREATEing. You still have the problem that replication is single threaded, so this doesn't scale your writes beyond one thread.
