Building Rdoc from source still works however.
but need to check out how often it builds (hooks?), and link it up from the homepage/readme/wiki so that people can find and use it
"If you’re the project owner, it’s even better; just add the following simple post-commit hook to your project’s settings in GitHub http://rdoc.info/projects/update and it’ll automatically regenerate documentation for you whenever you push to the remote."
I've just uninstalled all my formtastic gems, installed the latest gem and the -d option and the gem installed fine including rdoc documentation, so I'm going to close this issue.
speechless:~/ $ sudo gem install justinfrench-formtastic -d
Successfully installed justinfrench-formtastic-0.2.1
1 gem installed
Installing RDoc documentation for justinfrench-formtastic-0.2.1...