Cross-site Cookies and Login

I've scoured the forums and this seems to be a persistent issue for WPMU and domain mapping. Hopefully someone here can help me out.

I'm trying to have a single login page at, after logging in here, I want users to have access to all sites for commenting, viewing, etc. This works fine when my sites are all subdomains of ''. It doesn't work however, when domain mapping is applied because the cookies are set for So and treat the user as not logged in. Is there any way for all sites, including mapped domains, to check for the root domain cookie for authentication? Or perhaps checking for the root cookie and then setting the proper cookie of the current domain?