Jay, there are a few different reasons somebody can get blocked at AT&T, from what I can tell.
- A significant spike in sending volume or spam complaints.
- A significant spike in sending volume or spam complaints in the same network neighborhood as you (meaning other sending IP addresses in a /24 may have caused the blocking).
- Some really bad stuff is going on, sending some sort of affiliate spam or really, really unwanted stuff that they're able to identify and/or fingerprint through various means that they don't disclose.
Assuming the issue is #1 or #2, the way to resolve it is to submit that unblock request via AT&T's website. Alternately, if you've had no response after many days, you could try sending mail to postmaster at att.net. However, if there's a reason they're not responding, because they're busy, behind, or not able to assist, pinging them again via another method isn't likely to get you a response.
It's the holiday season right now, which means lots of people are on vacation and away from work. Maybe there's a backlog of unblocking requests awaiting review and approval at AT&T. Also keep in mind that ISPs don't view their postmaster teams as the treasured and necessary resource that they once did; lots of ISPs used to have whole teams of people managing these things, and in most cases, that has been reduced to a web form and some tiny part of some single person's job. Responding to blocking requests is just not a priority for most ISPs.
And if the issue is #3, then forget about it. They'll probably just go radio silent on you, and not respond at all. Most ISPs simply don't respond to inquiries about really bad stuff. I could only theorize as to why, but if it were me, I'd figure there's no point in helping the bad guy understand how we caught him. I know that could feel unfair, because what if you're not a bad guy, and you're given no opportunity to make your case. That's just the way the world works, sometimes.
I don't see any evidence to suggest that Spamhaus is used by AT&T, nor would I make any sort of assumption that your mail would or would not be delivered to AT&T subscribers based on a Spamhaus lookup.
AT&T also publishes a postmaster site at http://www.att.com/esupport/postmaster/. I strongly recommend reading all the recommendations they provide there as far as best practices and how to ensure your mail is delivered successfully.