normally you'd render the form again. to give the user a chance to correct his input. when redirecting you'll loose the users input

    def create
      if @user.save
        redirect_to(:register, :notice => 'Registration successfull. Check your email for activation instructions.')
      else
        render :register 
      end
    end

    def register
      @user = User.new
    end

besides that... why is the method called #register ? looks pretty much like #new to me ;)