I've tried installing both the Aptana plugin (I'm running Juno) and Aptana studio itself from http://www.aptana.com/products/studio3/download.
Just like with my previous experience with PyDev, I learned that I had to point Eclipse to my Ruby interpreter, but when I open my window > preferences dialog, there are no options for Ruby like what I've seen from tutorials and whatnot:
Even though Eclipse will allow me to create a new project, I don't see any option to create a Ruby source file: