Operation not permitted
Changing the UNIX wrx permissions with chmod did not work (no effect). Some googling revealed that the permissions are controlled by ACLs (access control lists). With Leopard these can be disabled for the whole disk with
sudo fsactctl -p /Volume/MyVolumeName -d
Now Snow Leopard does not have fsaclctl. But apparently you can just use the one you have from Leopard. You can just copy /usr/sbin/fsaclctl from one of your Leopard time machine backups to /usr/sbin of your Snow Leopard machine.
This post gave the hint: http://forums.macosxhints.com/showthread.php?t=105879