Syncing two (or more) PostgreSQL 9.1 clusters
There isn't currently anything in the PostgreSQL core which would help with that, but there is work in progress to do what you want that is likely to make it into version 9.3, which will probably be released in the summer of 2013.  If you can't wait for that, there are a number of solutions available from third parties. From what you describe, Bucardo sounds like it might be the best fit.

A new option, which recently hit production release 1.0, is Postgres-XC.  It is exciting technology, but if you have a stable production environment you might want to just keep an eye on it for now; it's pretty "bleeding edge" at the moment, and they haven't yet figured out how to incorporate all of the features of normal PostgreSQL in that environment.