Don't want to read through all the responses, so I don't know if this has already been said...but unless there were 2 Tobi at once, Itachi had to have met the Obito Tobi.
Because of the Kyuubi Attack shortly after Obito's death. About a year or two, actually. (Kakashi Gaiden occurred when Kakashi was 13, Obito was the same age; Kyuubi Attack happened 12-13 years before the start of the series; Kakashi was 27 at the start of the series, meaning he had to have been 14 or 15 at the time of the attack; Obito would have only been dead 2 years at the very most.)
The Tobi that attacked Konoha during that attack used Obito's Mangekyou abilities against Minato. Meaning that Tobi had to be Obito. More on this later.
Itachi met Tobi shortly before his massacre, roughly 5ish years before the series started. Meaning, Itachi met with that Tobi after Obito attacked the village. Unless Madara and Obito were both pretending to be Tobi at the same time, it had to be Obito.
I have a theory as to why it fits. It's only Obito's body. Real Obito is dead, and the body is possessed by someone much older and much more powerful. Assuming the body would have been modified for efficient use, it explains how Obito grew to adult size in a few years, how he gained so much ancient knowledge about how the Sealing Technique used to seal the Kyuubi works (He knew that it would be weakest during childbirth...someone Obito's age would have no way of knowing that), how he gained so much skill so quickly, why he keeps making comments that imply he's much older than Obito (telling Kushina that he's waited a very long time for the Kyuubi, when Obito only "died" a year or two ago, telling Konan he gave Nagato the Rinnegan and encouraged Yahiko to form Akatsuki, explains why he told Kabuto that he obtained Hashirama's cells during their battle AFTER Kabuto already showed him Madara's corpse), how he was controlling the Mizukage during the Bloody Mist, even though the Bloody Mist was ended by Zabuza when Obito was a child, it explains why Tobi doesn't care about his body parts all that much (his right arm is the most abused limb ever...Amaterasu, Nano-bugs, Konan's explosions, Naruto's Rasengan...none of which really seems to bother him that much), and finally, explains why he's so interested in Sasuke progressing his eyes-in Orochimaru-like fashion, he wants Sasuke's body/eyes.
And if it happens to be Izuna, it also explains why he hates the village (for betraying his big brother), and explains why he gets upset when people suggest ill of Izuna and Madara's relationship, and explains what he meant when he said the Rinnegan was his in the first place-Madara unlocked Rinnegan after advancing his EMS, which was obtained by transplanting Izuna's eyes, meaning the Rinnegan eyes Madara had were, in fact, Izuna's eyes. Finally, it would explain how Tobi was able to control the Kyuubi, a feat only Madara was supposed to be capable of-Izuna was said to be Madara's equal; meaning, if Madara could do it, Izuna should have been able to do it too.
Also, it being Izuna would draw a nice parallel between Madara and Izuna and Itachi and Sasuke, which, to me, sounds like something Kishimoto would do.
That, or Obito is a time-traveller. I personally prefer possession, since possession is already a theme firmly established in the Naruto universe, while Time Travel only happened in a movie, which doesn't count as canon.
I was beginning to think that too, kishi will probably take that route. Could also be that izuna and obito are sharing the body. Would explain why he has access to obito's memories.
I say don't jump to conclusions, we don't know yet if tobi is really obito. The certain things we know is that it is obito's eye and obito's+zetsu body.
@Zau: Yep, either all that stuff about Tobi are just plotholes that Kishi brought on himself, or he actually intends to fill those plotholes by revealing Tobi is actually someone else.
I like the thought of Tobi possibly being some sort of Uchiha-body possessor. Like the Uchiha-version of Orochimaru.
Or as you said, it could make sense if timetravelling was involved
so assuming there is a 'final product', oro's procedure was:
Body 1 --> Body 2 --> Body 3 --...> Final Product
isn't it easier and more prudent for survival if:
Body 1(with hashirama) --> Body 2(with hashirama) --> Body 3(with hashirama) --...> Final Product
orochimaru only changed bodies once every 3 years, and he picked his bodies carefully