Drupal 7 Single Sign On possibilities with the Domain Access Module The background:
We (currently) have 2000 domain names pointed at a single IP. Drupal inherently has the ability 
to use multisite, but for our particular Drupal 7 install we use a module named Domain Access to 
delegate sites. The gist is though that Domain Access will not allow cross site logins, but yeah, 
we would like our editors to log in once, and have access to all sites. 

The question:
Has anyone made any headway with coming up with a single sign on solution with Domain access to date 
for Drupal 7? I've been looking closely at two old modules, one no longer maintained (SSO for D6) and 
one still maintained (CAS). I'm worried about using CAS I am not sure if it will work the way I intend
it since Domain Access seems to change quite a bit of Drupal(ly) stuff.

We could potentially use facebook, openid, twitter option, but i thing not having able to offer visitors
or customers a unique id across a network of sites is too me a big loss of identity and privacy.

Are there any solutions that are known to date, or information on what to start looking into? Has anyone in
the Drupal community managed to make a solution that I have not been able to find? The only possiblitiy that
I've found was this thread on Domain Access SSO but am uncertain to what version it pertains to (Drupal. 
DA, SSO or otherwise). I tried contacting the author multiple times but never received a response. It looks 
like the "Solution" was to create a master table set with users and permissions, then share those across the 
domains. Unfortunetly (again) SSO on Drupal 7 is not an option.

Any opinions out there on the who what or why would be greatly appreciated, I honestly just need a start point 
to get the ball rolling. Thanks everyone
See: Comparison of Single Sign On (SSO) Modules

Bakery supports SSO for sites with the same domain (site1.example.com, site2.example.com). Have not tried CAS as you noted above it is an option. You could also use an oauth server to authenticate across domains.