Upgrade mysql from 5.0 to 5.5 in master-master configuration leads to broken replication
Since all the data is MyISAM, you should expect table locks rather frequently since INSERT , UPDATE, and DELETE command issues a full table lock.

If you have a lot of tables that use auto increment keys, you should add the following

Server1

[mysqld]
auto_increment_increment = 5
auto_increment_offset = 1


Server2

[mysqld]
auto_increment_increment = 5
auto_increment_offset = 2


This will prevent PRIMARY KEY collisions with autoincrement columns

I have other suggestions in my post I have been tasked with Mysql Master-Master replication?