IGN reports, citing "multiple reliable sources." Telltale told Joystiq, "We get these sorts of rumors about what we could be working on next cropping up all the time. Officially we have no comment."
Telltale successfully transposed The Walking Dead comic series into video game form, and it's currently working on The Wolf Among Us, an adaptation of the Fables comic series. Episode 2 of The Wolf Among Us is due next, and it's set up to hit a high bar.
If this Game of Thrones project is legit, we hope it includes a "Slap The Royal Brat" mini-game. That one's free, Telltale.