I've gone through the code, and altered about 200 instances of wfQuery() to specify whether it is reading or writing. No warranty expressed or implied. It's in CVS.

Reading: wfQuery( $sql, DB_READ, $fname );

Writing: wfQuery( $sql, DB_WRITE, $fname );

Currently, wfQuery doesn't do anything with this information, but the idea is to have a list of database servers in LocalSettings.php. Using 0 for the extra parameter would connect to the first server on the list, and send the query. DB_WRITE (-2) would use whatever the current master is, presumably this can be changed if the master goes down. DB_READ (-1) would invoke some sort of load balancer to decide where to send the query.

I haven't really looked into the synchronisation issues yet, but I have a feeling it won't be a big problem.

-- Tim Starling.