ACID-compliance of MySQL master-master replication?
MySQL replication is not ACID compliant since it is asynchronous.  There are no guarantees that queries executed on one server will be committed on the other.  In fact, the master doesn't even know about the slave except that it's pulling binary logs.  The binary logs themselves "might" be considered ACID compliant when sync_binlog=1 is on, but one server does not know if the other server has committed the change.

If you truly need something close to ACID compliance in your replication, what you want is synchronous replication, which is provided by products like Tungsten Replicator, Galera, and Percona Xtradb Cluster (which is Galera + Percona Server), and MySQL Cluster.  I'm not sure if they fit a strict definition of ACID compliance, but they are certainly a lot closer.  MySQL 5.5 has something called semi-synchronous replication, but don't be fooled...it's still asynchronous. 

You can improve the reliability of MySQL replication by doing a few things:


Use Row-based Replication (RBR) by setting binlog_format=ROW in my.cnf.  This will replicate the entire row for every change made instead of replicating the SQL statement that made the change (as with statement-based replication, [SBR]).  That minimizes the chances that a statement executes differently between servers if there are small discrepancies already.
Run pt-table-checksum frequently to verify the consistency between master and slaves.  If there are certain tables that are more important, run it on those tables more frequently.
Set up monitoring that checks the output of pt-table-checksum and alerts you when there are discrepancies.  You can use pmp-check-pt-table-checksum from the Percona MySQL Nagios Plugins if you use nagios.  Otherwise, it's just a simple query to determine this information. 
If you have discrepancies, use pt-table-sync to resolve them.  Read the documentation carefully, as you can shoot yourself in the foot relatively easily with this tool.