What kind of association is :organizations? Does it work if you specify :as => :select?
There's spec coverage for the following belongs_to select, date, time and datetime inputs:
f.input(:author, :as => :select, :include_blank => true)
f.input(:created_at, :as => :date, :include_blank => true)
f.input(:created_at, :as => :time, :include_blank => true)
f.input(:created_at, :as => :datetime, :include_blank => true)
My guess is that organizations is not a belongs_to association, right? If it's a :has_many or :has_and_belongs_to_many association, Formtastic will try to do checkboxes or a multi-select. In the case of a multi-select, obviously it makes no sense to have a blank line in there (you just don't select any of the items).
Hope this helps, please post some more details about the models and associations in question.