Help debugging lost session (rails + facebook)

I have an app that is simultaneously using restful_authentication and
rfacebook (0.9.8).
(I can't use facebooker because it is completely incompatible with many
other modules).
restful_authentication works just fine along side those logging in using
facebook on the external site...
where things go south is within the facebook canvas, but I think this is
a rails session problem, hence I am posting here for help.

I get a callback showing facebook login worked:
Processing SessionsController#create_from_facebook  [GET]
  Session ID: ZZZ
  Parameters: {"fb_sig_time"=>"1216501629.9969",
"fb_sig_in_iframe"=>"1", "fb_sig"=>"e5e413db3e28e76053e6cb29e004e
94c", "action"=>"create_from_facebook", "fb_sig_session_key"=>"yyy-xxx",
"controller"=>
"sessions", "fb_sig_expires"=>"0", "fb_sig_added"=>"1",
"fb_sig_api_key"=>"", "fb_
sig_profile_update_time"=>"1215049916", "fb_sig_user"=>"xxx"}
** RFACEBOOK INFO: Activated session from inside the canvas (user=xxx,
session_key=yyy-xxx, expires=0)

the user_id is stuffed into the current session at this point, and
current_user is set.

the app then redirects, and it appears the session is completely
switched (and lost) - i.e. at least the user_id stored is gone, and the
app gets into a infinite loop of redirecting back to login: