Devise login with user or admin models and Basecamp style subdomains

I have separate models for Devise users and admins. I am also using
Basecamp style subdomains. Everything is working well except for a few
controllers and actions where I need to be able to authenticate as
either a user or as an admin.

Currently I have authenticate_user! set in my application_controller.rb
and I am skipping it with skip_before_filter for those controllers and
actions that only admins should have access to.

Unfortunately I cannot simply specify the authentication requirement on
each controller as I will still need some controllers and action to be
access by both a User or an Admin.

I have tried a few things to no avail. It seems that if I move the
authenticate_user! and authenticate_admin! into some sort of subdomain
detection logic it fails to process. Basically:

current_subdomain = request.subdomains.first
if current_subdomain == 'admin'
authenticate_admin!
else
authenticate_user!
end

I was, at one point, able to get it to attempt authentication but for
some reason it was failing to except the session controller from needing
authentication which resulted in a redirection loop (a first for me with
Ruby!).

I realize that I could add a field to my User that denotes admin status,
but the application requires a greater separation of powers between User
and Admin than that will allow, except for a few controllers and
actions.

Ruby 1.9.2
Rails 3.0.3
Devise 1.1.3