The answer is, of course, that depends.
Currently Postgresql Inc. puts out a version of rserv that is non-open and commercial that works well. It is a synchronous clustering system, and fairly simple, but reportedly works well. Is is a single master multi-slave setup. www.pgsql.com
Postgres-R is in the works, which, when it is finished, will be capable of synchronous or asynchronous clustering, as well as multiple master / multiple slave.