I have a problem with an CMS i have build with CodeIgniter. If you loggin on /admin and you are redirected to the website, then the session class is empty. I have traced this problem to double cookies that are created for the www domain and non www. The non www has all the login info, but the www has no data.
Setting the cookie path is not as easy as it sounds, because the application is symlinkd to multiple websites.