Ubuntu 8.04 LTS (Long Term Support) actually include Mysql Server 5.0 only.
In order to install the version 5.1 you need to add additionals DEB repository to your APT’s config.
For instance append those two new repository to your /etc/apt/sources.list files:
deb http://ppa.launchpad.net/monty/ubuntu gutsy main universe restricted multiverse deb http://ppa.launchpad.net/smurf/ubuntu gutsy main universe restricted multiverse
Then you can install it:
apt-get update apt-get install mysql-server-5.1
That’s it, have fun.
How to install Mysql server 5.1 in Ubuntu 8.04 LTS,