Hi-Rez Studios has pinned the Tribes: Ascend closed beta for a November 4, 2011 commencement. As the name would imply, this is going to be rolling out on an invitation-only basis, though anyone who pays for the VIP Starter Bundle ($29.99) is promised an open spot for this and any future beta phases.
Got questions? Hi-Rez has answers, in the form of a convenient FAQ section. We're so trained at this point to sign up for official-game-site accounts that I don't even need to nudge you in that direction.