One of the weirdest bugs I have ever come across.
After spending hours in the network closet, systematically unplugging each device to see if it had any effect on the issue, I finally found it - it was a printer who's Bonjour name was “Mörkö”. The problem disappeared after renaming it to “Morko”. So I filed a bug report.
It turned out Apple saw this as a security issue and so they took it very seriously and I'm happy to report that it's been resolved in 10.6.4:
I think this might be the first time one of my bug reports has actually been fixed. It was really nice to see Apple take this as seriously as they did and it actually motivated me to keep reporting bugs in the future (I had kinda given up on that after all those “known issues”). Of course, having your and your fledgling company's name mentioned on apple.com never hurts either (even though probably 9 people in the world actually read the detailed contents of Security Updates). :-)