In real life however many people including me end up with an error message like this:
Missing requirement: Android Development Tools 12.0.0.v201106281929-138431
requires 'org.eclipse.gef 0.0.0' but it could not be found
I found some post that advised to install Eclipse Java EE Developer Tools. I did that, but it didn't help (no big surprise). I then turned on my logic and googled for "org.eclipse.gef" alone. It was defined as Eclipse GEF (Graphical Editing Framework), which happened to be a part of Eclipse Graphical Modeling Framework.
So, to overcome the above error if you have it, first make sure that you have the correct update site for your version of Eclipse enabled in the list of Available Software Sites. For instance for Galileo that should be http://download.eclipse.org/releases/galileo/. Then select that site and install Graphical Modeling Framework.
After that follow the Android's howto and it should work for you.
Hope this helps.