Rack-P3p is rack middleware for inserting P3P headers into apps. This will allow your application to serve cookies to IE when in an iframe. It will also pull off the cookie on 304 responses, since IE does not like the cookie then.
To use Rack-P3p in rails, do the following. Anything else and we'll assume you're smart enough to figure it out :-)
Add rack-p3p to your gemfile:
Add it to your middleware stack in config/application.rb:
config.middleware.insert_before ActionDispatch::Session::CookieStore, Rack::P3p
The code for this middleware was shamelessly ripped from a rubyonrails-core post from Robert Vogel. Thanks, Rob!