Slony-I is a terrible piece of software; it's incredibly difficult to administer, requires full table locking across the entire data set (EVERY SINGLE TABLE!) to do DDL changes, and occasionally breaks in mysterious ways which can lead to data corruption.
I haven't used PGCluster, but it's synchronous multi-master and thus inherently fails to scale out as well as async systems like Slony-I and Londiste. Probably great in circumstances with only 2-3 db boxes though.
Londiste, on the other hand, is awesome, and with the 3.0 release now has hot backups. Simple to administer, rock solid, and has really important features like live DDLs. Demand Londiste and the SkyTools package! Don't settle for less!
