autocmd FileType ruby,eruby set omnifunc=rubycomplete#Complete
  autocmd FileType ruby,eruby let g:rubycomplete_buffer_loading = 1
  autocmd FileType ruby,eruby let g:rubycomplete_rails = 1
  autocmd FileType ruby,eruby let g:rubycomplete_classes_in_global = 1

and for Supertab

  let g:SuperTabDefaultCompletionType = "<c-x><c-o>"

or

  autocmd FileType ruby let g:SuperTabDefaultCompletionType = "context"

That way you will get omni completion with supertab.

I compiled this from http://www.cuberick.com/2008/10/ruby-autocomplete-in-vim.htm... and some SO answers.