install pre-requisite packages (yes, vim and wget are optional):
aptitude install apache2 libapache2-mod-php5 mysql-server \ php5-mysql php5-mcrypt php5-gd openssl vim wget
edit /etc/apache2/sites-enabled/000-default
<Directory /var/www/> Options Indexes FollowSymLinks MultiViews AllowOverride FileInfo Options Limit
edit /etc/php5/apache2/php.ini
magic_quotes_gpc = Off
install roundcube (or download the latest):
cd /var/www wget "http://voxel.dl.sourceforge.net/sourceforge/roundcubemail/roundcubemail-0.2-beta2.tar.gz" tar -xzf roundcubemail-0.2-beta2.tar.gz ln -s roundcubemail-0.2-beta2 roundcube
now just complete the web based install at:
http://SERVER/roundcube/installer