With one of the many third-party replication packages that have grown around PostgreSQL (specifically, Bucardo v5 which is in beta right now), you can do multiple — as in more than two — writeable masters, though, as the linked page notes, the replication is asynchronous. With older versions of Bucardo, you can do two masters — again, asynchronously (though you can technically get more than two if you want to engage in some significant configuration gymnastics).
What they're calling "synchronous multi-master" replication is actually implemented using "two-phase commit", which isn't replication per se, but rather application code using a database feature that allows specifically crafted database interactions to be written to multiple databases, and only successfully committing if the participating nodes agree that they've all committed.