AA has changed the login page for both Gogo planes and Viasat planes to aainflight.com. I was on two 737-800s recently that had Viasat wifi, and no luck as a T-Mobile user. It's good to know that a provision to log in with a T-Mobile account still exists on the Gogo-enabled planes.

Interestingly enough, the reason AA has a bunch of Viasat planes is because they had a "shiny new thing" clause in their contract with Gogo that let them break the contract when the faster Viasat technology came along.