We upgraded from PostgreSQL 7.4 to 9.0 at work over the winter break. I was much easier than the tests we did moving from 7.4 to 8.x Everything went fairly smoothly. The only trick has been with dblinks.

Evidently, PostgreSQL changed how they work, and we had to modify some things to get it working.

It's nice to be mostly rid of the unupgradable P3 733MHz single processor box that the 7.4 server was running on. Although, that box has been our main DB server for the website and the webapps we've written since before I started here in 2002!

That's a pretty good testament for Linux and PostgreSQL.