After upgrading to Snow Leopard I had quite a bit of trouble with my MacPorts installation, since most builds just failed with configure errors. After the obvious first steps of installing a newer version for SL and making sure I have the most recent XCode, I was a bit stumped until I finally found this site:

Migrating a MacPorts install to a new major OS version or CPU architecture

After finding this guide – which btw really solved my problems – I felt a bit stupid, but since I didn’t have to do anything after the Tiger→Leopard upgrade, I really didn’t expect this.