Howtoforge posted a guide about setting up master-master replication with MySQL 5 on Debian Etch

Since version 5, MySQL comes with built-in support for master-master replication, solving the problem that can happen with self-generated keys. In former MySQL versions, the problem with master-master replication was that conflicts arose immediately if node A and node B both inserted an auto-incrementing key on the same table. The advantages of master-master replication over the traditional master-slave replication are that you do not have to modify your applications to make write accesses only to the master, and that it is easier to provide high-availability because if the master fails, you still have the other master.

Setting Up Master-Master Replication With MySQL 5 On Debian Etch