AT&T

AT&T owns a number of domains, including SBC and Bellsouth. They have been integrating their various acquisitions into a standard mail processing system, but as of Jan 2010, the integration is not complete. Some AT&T domains also use Yahoo filtering rules to send mail to the bulk folder

Delivering to
There are no published limits  for connection or sending limits. I have seen claims of limits of 2 connections per IP and 30 connections per IP, but there has been no data presented for either claim.

Postmaster pages
AT&T provides postmaster pages with information about bounce codes and blocking. They also provide information for other receiving sites, including a list of their outgoing SMTP servers. All of the AT&T support pages are behind a location filter. Users will need to provide a zip code to access the pages. This does not affect the information provided, so any valid zip code will do. If you are outside the US, you can use 94301 (Palo Alto, CA) as your zip.

Feedback loop
AT&T does not offer a FBL.

Whitelist
AT&T does not offer a whitelist.

Certification Services
AT&T uses the Return Path Certified whitelist in mail delivery decisions. Mail from IPs on the Return Path Certified list will see better delivery at AT&T domains.

Support for Senders
AT&T provides minimal support for senders. Senders who have email blocked at AT&T can follow the URL in the rejection message, or visit their blocking page

Known spamfilters
Based on the information provided on their postmaster website it appears AT&T is using some of the Spamhaus and the Symantec blocklist. The specifics of this usage has not been published.