Right, I'm bored and some idiot on another board is spouting off about all the proof behind Obito being Tobi, so here comes my own theory.
As we all know, Uchiha ninja were considered elite. It stands to reason that a fair few of them got assigned to various teams under jounin, a few of them likely made it in to ANBU and maybe some of them even lead their own cell, no? Does it not also stand to reason that several of them would have been out on missions at the time of the slaughter?
Perhaps one of these was a 20-something chap by the name of Tobi. Uchiha Tobi. Crazy, right? Tobi's real name could be Tobi, what are the odds? -__- Now I don't know all of the details, but I do know that he had spiky black hair, had bolts & pins in his left side and wore an orange mask with a swirl pattern focusing on the hole in his mask corresponding to his right eye.
Seriously, why does Obito have to be anybody? Why does every new character introduced have to have some link to every bloody character in the series?