As a WWE Network subscriber I received the following emails just moments ago. I saw that this was rumored on the A1-Wrestling Forums the other day, but had not heard it elsewhere. Here's the official confirmation of another free month for new subscribers. I believe the last one was in November.
It makes the inclusion of Sting at Fast Lane kind of interesting. I figured that was there to maintain the Royal Rumble subscribers and to avoid a cratering of subscriber count between Rumble and Mania. OTOH, I suppose that giving something of that value away during a free month might motivate people to stay on after the free month expires.