It's not clearly stated, but if not he's probably trapped together with Gallifrey. He was consuming his lifeforce very quickly to attack Rassilon in "The End of Time", so it's possible the Master is dead.
But considering the nature of the storyline, the Master, like the Daleks and the Cybermen, will always find some way to come back.
Even if the Master is dead, you can't trust him to stay that way.
As far as we can tell, he is trapped with Gallifrey. However, the Master has been resurrected before, and we can't really rule him out as impossible to return, although I sense it is unlikely that he will return still portrayed by John Simm. Simm is now older and persuing his other careers; new doors opened for him when he got the lead role as Sam Tyler in Life on Mars.
However, a personal plea;
Please come back John, I fancy you. :D