inverse_of not supported in Rails 3 for has_many?

I was looking through the association classes in the rails 3 source and
noticed that we aren't supporting bi-directional associations with
has_many and belongs_to:

        # NOTE - for now, we're only supporting inverse setting from
belongs_to back onto
        # has_one associations.
        def we_can_set_the_inverse_on_this?(record)
          @reflection.has_inverse? && @reflection.inverse_of.macro ==

Why wasn't that implemented, it doesn't seems much more difficult to
than has_one and belongs_to.

I'm running rails 3.0.5.