I went through the Naruto character biographies
here (not sure if that includes everyone though, anybody got a better source?) and I'd have to say I'm no closer to getting at who Tobi is. Nobody fresh comes to mind. Infact im starting to think we've never see this character before.
Although one thing i did notice was Shisui's appearance being eerily similar to that of the sages elder son. So that shall be my flavour of the month for my Tobi = theories.