In #ruby IRC, ryanf, who likes penguins (according to shevy), recommended using rvm to install all the ruby junk. Taking care of rvm prerequisites, rvm installation is like so:
apt-get install curl git-core subversion
bash < <(curl -s https://raw.github.com/wayneeseguin/rvm/master/binscripts/rvm-installer )
According to Redmine's tedious compatibility requirements, for Redmine 1.2.x install a) Ruby 1.8.7, b) Rails 2.3.11, c) RubyGems 1.5.3 (recommended by ryanf), d) Rack 1.1.1, e) Rake 0.8.7, f) I18n 0.4.2, g) MySQL C bindins for Ruby:
# install ruby 1.8.7
rvm install 1.8.7
# switch gems to the version desired
rvm rubygems 1.5.3
# install rails using gem; this should also install Rack, Rake, and I18n
gem install rails -v=2.3.11
# finally, install mysql c bindings for ruby
gem install mysql