* PHP session storage must be shared. Either set this up on a networked
filesystem or set up memcached and enable the memcached session storage
option.
It is also possible to use PHP's session_set_save_handler function to store
sessions elsewhere, say in a database table. See
http://php.net/session_set_save_handler for more info.
In response to the original question, you are probably also curious about
database clustering. There has been some talk of this on the list in the
past. However, I don't recall anybody ever saying they have set up a MySQL
cluster with MediaWiki. However, it should be possible to set up MySQL with
multi-master replication. Hopefully MySQL 5.1 will handle all of the
current shortcomings, but we will have to wait and see...
Gregory Szorc
gregory.szorc(a)gmail.com