WWE Network is available on Apple TV as of this morning. You will have to refresh your Apple TV to see the app. You can subscribe to the Network through Apple TV using your iTunes account.
Up until this morning, Apple TV had not been listed on the list of devices that would support the Network. It's likely that WWE was trying to get users to sign up through their website instead of Apple TV, since Apple takes a larger cut (around 50%) of each subscription.
It should be noted that if you register through Apple TV, subscriptions will be available in six-month blocks. So you will be charged $59.99 (plus applicable tax) for the six month subscription period up front after the trial period ends.
If you were to sign up on WWE.com, you would be billed in monthly installments of $9.99 / month (plus applicable tax) with a six month commitment. So if you signed up this month, and decided to cancel after WrestleMania, you would still be charged $9.99 for the four months following WrestleMania.
You can check out photos of the WWE Network on Apple TV below: