There is an error in the current version of Samba on Ubuntu Karmic you find in the samba log file (/var/log/samba/log.smbd) this message:
[2009/12/07 12:26:26, 0] smbd/server.c:456(smbd_open_one_socket) smbd_open_once_socket: open_socket_in: Address already in use
The best solution is a downgrade to version 3.3.2:
wget http://security.ubuntu.com/ubuntu/pool/main/s/samba/samba_3.3.2-1ubuntu3.2_amd64.deb dpkg -i --force-all samba_3.3.2-1ubuntu3.2_amd64.deb
Update:
Problem solved (Version 3.4.0)