Configure Rails with MySQL under Ubuntu – Socket Problem

I am currently developing a Ruby on Rails (RoR) application under Ubuntu. I am using MySQL as the database and Netbeans 6.1 for developing. When I tried to access the database, I got following error:
No such file or directory – /tmp/mysql.sock
Ok, pretty straight forward. Since in Ubuntu/Debian the mysql.sock is located at /var/run/mysqld/mysqld.sock, I added
socket: [...]