On my computer yesterday I installed 12.04 and applied all updates. I have installed lamp on this system using following command sudo apt-get install lamp-server^ phpmyadmin . I created a new user from phpmyadmin and given that user all the global privileges, but I am not able to login using that new user, I have also restarted the mysql service, reload the privileges, but still its not working. It used to work fine in Ubuntu 10.04.
Mysql is a bit odd in the fact that you if you want to login from that server only it is 'username'@'localhost' but if you need to login in from another server also you have to add a second user entry 'username'@'ipaddress' or 'username'@'%' for any host (apart from local host) and then grant privileges user each variation.