Update: The AT&T network is back to normal now.  It looks like the problem was an ISP (Telefonica) injecting some bogus BGP routes, that caused customers of some other ISPs to be unable to reach AT&T (and some AT&T customers to be unable to get return traffic from these ISPs).  There does not appear to have been any actual problem on the AT&T network.

"On Sept 9th, AT&T customers experienced an inability to reach certain Internet sites. Initial investigation indicated packet loss to the AT&T DNS resolvers. Further investigation by the AT&T Common Backbone Engineers determined the issue to be an erroneous advertisement of the AT&T 12.0.0.0/8 block by another Internet Service Provider (ISP). Once the advertisement was withdrawn, correct routing information was propagated through our peers' networks." - AT&T Email to Customers

According to notes from users, and Keynote, AT&T is currently experiencing outages across its network. We do not have any details right now. This outage may affect the latency or reachability for a large number of sites.

AT&T's own network status page shows no problems.