With the introduction of PostgreSQL 8.4, Bruce Momjian, a significant core developer, contributed a tool that can actually upgrade an entire database cluster in place. The time required is essentially only that necessary to copy the data files from the old installation to the new one. On a quick RAID system, this can be an order of magnitude faster than a dump/restore. The main drawback is similar to Slony: disk space must effectively be doubled for this upgrade method.
I was fighting with packaging some software at work, trying to produce a workable RPM package to replace the manually installed kludge currently polluting one of our servers, and discovered the
--spec-only option to the