Seeing the information posted by Cro i think Rob is okay but he just needed a break. A message would have been nice so we knew he is okay. Its his decision though. Im sure we will see him back sooner or later.
ITST (or more community sites like ITST in general) can be like a vacuum absorbing all your time. When you feel you need to change this it can also be hard to turn your back on it because of all the time you spent on it (bit of a paradox there). So i do understand if his disappearance is because of that he wanted to make a hard break. Even when its out of character.