AT&T Locker is definitely encrypted and secure, you can tell it's using an encryption by an "s" after "http" in the address bar, and sometimes there might be a padlock symbol. My browser reports that is uses TLS 1.0 Encryption, which is very similar to SSL 3.0. It's considered to be slightly better than SSL 3.0, some call it "SSL 3.1" due to their similarities and a common origin.
Thanks for the reply. Although I'd assumed a secure connection, I'm more concerned about whether or not the information is encrypted at-rest on the ATT server. The ATT rep I spoke to was unable to answer the question, but was quite cheerful about her ignorance, and had no interest in finding out the answer. Simply put, if ATT doesn't use AES encryption at-rest, I'm not using it.