That is also my theory but it becomes quite unstable if you take the Library Two parter in accountant. The way River died showed compassion and love for the doctor something the Master would not show (explicitly) and he would certainly not read bed time stories for little kids.Sam G said:Okay, how about this: River Song is the Master.
No, seriously, it makes sense. He's the only person in the universe I can imagine finding out the Doctor's real name, and so once he regenerated (as a woman) he decided to screw with the Doctor's head by pretending to be his wife from the future. Hah, that wacky Master, him and his antics.
So either River must have forgotten that she is the Master (we had this already) or the Master had a serious turn to be at least a saint like the doctor whereas I believe the Master works as an Anti-Hero at best.