Default package can’t support mysql 5.1.
There are some solutions to solve this problem. For example, You can install the non-RPM packages which can be downloaded from official web(www.mysql.org). But, my choice is to add some other package sources into source.list.
Edit file :
#vi /etc/apt/sources.list
add new line :
deb http://ppa.launchpad.net/monty/ubuntu gutsy main universe restricted multiverse
deb http://ppa.launchpad.net/smurf/ubuntu gutsy main universe restricted multiverse
save and exit
Update & Upgrade
#apt-get update
#apt-get upgrade
Install MySQL 5.1
#apt-get install mysql-server-5.1