There was a short thread in March 08 about how the format param is always reset 
to fbml, preventing the use of respond_to 
(http://rubyforge.org/pipermail/facebooker-talk/2008-March/000464.html). I am 
experiencing this issue now.
As a matter of taste I would rather use
formatted_..._url(:format => :json)
and
respond_to do |format|
   format.json { ... }
end
over request.xhr?. Is this just not possible with Facebooker?
Alan.
-- 
<a href="http://wikimediafoundation.org/wiki/Donate/en"><img
border="0"
alt="Wikipedia Affiliate Button" 
src="http://upload.wikimedia.org/wikipedia/foundation/5/58/2008_fundraiser_micro_button-en.png"
/></a>