UPDATE: Just use RVM or install ruby-enterprise-edition, thats far easier …

Just writing this because it took a lot of time to find a working tutorial
Ubuntu Hardy(8.04) has 1.8.6 installed, and Ibex(8.10) already comes with 1.8.7.

sudo apt-get install build-essential libssl-dev libreadline5-dev zlib1g-dev
wget ftp://ftp.ruby-lang.org/pub/ruby/1.8/ruby-1.8.7-p72.tar.gz
tar zxvf ruby-1.8.7-*
cd ruby-1.8.7-*
./configure --prefix=/usr/local --with-openssl-dir=/usr --with-readline-dir=/usr --with-zlib-dir=/usr
make
sudo make install

Hope it helps but so far did not work out for me, when using /usr/bin/local/ruby xxx.rb rubygems are not found, and when installed using –prefix=/usr , openssl is broken, since it is the 1.8.6 version and not even manually installing ruby-1.8.7 openssl seemed to work…