Facebook just open sourced a new extension for +Mercurial
called hgsql. It allows you to do horizontal scaling of your Mercurial servers horizontally to better distribute the load.
It has always been easy to create read-replicas with Mercurial (or any other DVCS) since you can just setup some servers with mirrors of your main repository. To avoid multiple heads, though, writes would still have to go to a single master server. With the hgsql extension, you can now have multiple such masters. They synchronize using a MySQL database, which (assuming I understood the architecture correct) then becomes the central point of failure. I guess that is still an advantage since reads are handled by the Mercurial servers and there are standard techniques (such as replication) to avoid relying on the availability of a single MySQL server.