Below are listed some links that are helpful in understanding multi-master database replication, and some of the challenges involved in it.

• Multi-master Replication Overview - Describing methods of replication, including details of implementations in popular databases and middleware solutions.
• Multi-Master Replication Challenges - Describes the difficulties encountered in a mult-master replication scenario from an Oracle point of view.
• Replication, The Next Generation of Distributed Database Technology - Lengthy article on database replication technology covering different vendor approaches and difficulties involved.
• Database Replication - DBMS Magazine article by Charles Thompson comparing three leading DBMS vendors' approaches to replication.

These, and links to other replication technologies, can be found on the Open Directory Project (ODP) at

http://www.dmoz.org/Computers/Software/Databases/Replication/

and for PostgreSQL-specific stuff,

http://www.dmoz.org/Computers/Software/Databases/PostgreSQL/Replication/

As you can see, there isn't a whole lot out there, but it's a growing segment of the software industry, for sure.