MySql 5.5 in ubuntu repository
There are .deb packages available. Using the ones from http://people.debian.org/~nobse/mysql-5.5/:

sudo su -
wget http://people.debian.org/~nobse/mysql-5.5/mysql-common_5.5.13-2_all.deb
wget http://people.debian.org/~nobse/mysql-5.5/mysql-server_5.5_5.5.13-2_amd64.deb

dpkg -i mysql-common_5.5.13-2_all.deb
dpkg -i mysql-server_5.5_5.5.13-2_amd64.deb


The first dpkg execution may ask for you to install dependencies. If you're installing on a 32-bit system, alter the URLs accordingly.

Latest .debs are available on http://dev.mysql.com/downloads/mysql/#downloads - Select "Debian Linux".