I know that I'm some months late on upgrading, but for a production system I prefer to let things cool down a little before attempting the system upgrade. However, even with this methodology, there may be still some problems after finishing, and I have found two of them today, after upgrading to karmic koala version of Ubuntu (9.10 for number-centric fans).
The first one was a problem with mysql-client, which remained in 5.0 version for some reason. I was able to resolve this issue by manually installing version 5.1:
aptitude install mysql-client-5.1
The second problem had to do with updating vnstat's database ('vnstat -u'). After testing various ideas, I came up with no other solution than deleting the previous databases and recreating new ones with the following commands:
rm /var/lib/vnstat/eth* /var/lib/vnstat/.eth*
vnstat -u -i eth0
vnstat -u -i eth1
I hope that there will be no other surprises (until the next upgrade of course)