Right on top of my YaST Installer wishlist would be a "sort by install date" option in order to easily revert what yesterday's uncautious update screwed up. At least I found out where the installer log is: /var/log/YaST2/y2logRPM.
sudo tail -n 100 /var/log/YaST2/y2logRPM (100 last lines) ought to give you a hint about the most recent changes.
Note: Doesn't work for SuSE 11.1. Try
# sudo tail -n 100 /var/log/zypp/history
If you want to compare when you installed which version of a certain package:
# sudo cat /var/log/zypp/history | grep -i <packagename>