Rails - Jun 2006 - habtm and multiple-option select boxes

I''ve got a fairly basic has_and_belongs_to_many setup, with a Business 
model and a Category model.  In the admin functionality for the Business, 
I''d like to have a multiple select box showing all of the Categories,
with
included ones selected, etc.

I''m stumped as to how to do it, though - it seems like there should be
an
easy way, but I''m missing it looking over the docs.  I can''t
do the
canonical:

  select("business", "category_id",
Category.find_all.collect {|c| [c.name, c.id ] }, { :include_blank => true })

Because there''s no business.category_id due to the HABTM.

What''s the standard way to accomplish this?

Thanks,
Wade