I used a script to install ImageMagick http://github.com/masterkain/ImageMagick-sl
After a while, I got ImageMagick installed.
Then I ran sudo gem install rm
Initially, I came around to installing rmagick because it seemed to be a dependency when I tried installing libgtk2.0-dev. I was trying to install that package, because the RGtk2.0 is a requirement for using R package called rattle.
Anyway, none of the above solutions worked for me in installing in my Ubuntu 12.04. Even running sudo apt-get update first didn't seem to help, nor this related posting: Installing rmagick on Ubuntu
I eventually found what works is to troubleshoot using sudo aptitude rather than apt-get.
https://askubuntu.com/questions/223237/unable-to-correct-problems-you-have-held-broken-packages
I recommend the technique of using aptitude to work through the package installation issue.