Thanks for your comment. I must say we are currently in the process to moving to a master - slave architecture. We are having too much trouble with the master - master setup.
About the cache_* tables: yes, at the moment I've written this blogpost, we didn't have a notice of memcache. Since a month, we are running memcache for all cache tables and sessions.
About mongodb: thanks, interesting
About syncing files: yes, you could do that, but the problem is we really wanted to seperate the whole system. We don't want to have a single-point-of-failure. If the "same harddrive" fails, your site will not be available.
The writeup is written for Drupal 6. But after reading your comment, I consider writing a second blogpost with contra's about master - master and ... most important ... our idea of the new setup. We are also considering the use of a common NAS drive where all files are stored. So we do have a single-point-of-failure on that one, but ... the chances the NAS falls down are so little ... It's better to keep the files directory clean