After finally getting a backup server up and running, I could finally proceed with an upgrade of our primary servers. The upgrade path from Squeeze to Wheezy is pretty much smooth, mostly you just need to merge any updates into your config files, which is usually no big deal. Well, except for php.ini, which was about as painful as writing PHP code.
Anyway, the award for the most difficult package to migrate from Squeeze to Wheezy goes to... Postgres. In Squeeze, its version is 8.4; in Wheezy we have 9.1. Read on to find out what you do not want to do and, hopefully, what you might want to do as well.