Why Sasuke will join Akatsuki [it's a prediction, like it or not]
There has been some exposition on Sasuke eventually joining the Akatsuki club. But not just Sasuke alone, but team Hebi as a whole. Now look at what we have? Since the second part of the manga started, Akatsuki has suffered some casualties. Starting out with the loss of Sasori (though Tobi got his spot) followed up by Hidan and Kakuzu made nine members turn into seven. Recently Deidara went suicidal and Tobi (with Pain's help) faked his death, leaving 5 members behind. Pain is aware of this loss of members, and as he and Tobi are expecting Sasuke to surpass Itachi, they want this new Sharingan as part of the group.
Pain are not yet willing to give up on the idea of Akatsuki and he needs to recruit S-rank ninjas to achieve his goal. Though as Tobi has convinced him to expect (and let happen) Itachi's defeat, they know that Sasuke and team Hebi will come after Tobi himself - because he will appear to him during/after the Sasuke and Itachi encounter. This will help Sasuke realize that his mission is not complete even after Itachi's death and he will have to track down Tobi and kill also him. But of course, he will track Tobi to the Hidden Rain.
Now, why is it that Tobi will become Sasuke's new target? After Itachi told Sasuke to travel to the hideout (the first time, that is), he read about the true past of the Uchiha clan - the part involving Madara summoning the Kyuubi to attack the village. Originally, the secret hideout are only revealed to the successful Uchiha ninjas, those who has mastered the first stage of the Sharingan and is carrying out their duties as a Konoha ninja. Itachi knew that Madara was still alive, but Sasuke never got to know. After Itachi had mastered the Mangekyou Sharingan, he lost all hopes for the Uchiha clan as the true potential of their blood line limit was taboo; and because of it's dangers with Madara's past, it was forbidden to achieve. Itachi joined the Akatsuki club which he knew had a close connection to Madara himself (he knew this because he saw it with his Sharingan in the eyes of Pain). He quickly realized that he couldn't surpass Madara with ambition alone, the only way to surpass him was by revenge. That's why Itachi killed the clan (which he still had no respect for anymore) and made Sasuke into an avenger.
Itachi will taunt Sasuke through their encounter and Tobi will look over them from a distance. As Sasuke proves his potential by defeating his brother (possibly killing him), Tobi will appear and reveal his true identity and his true goals (don't ask me why but villains tend to do that in shonen). Sasuke gets to know that Itachi killed the clan to empower Sasuke, and still enraged at his brother for the genocide he decides that Tobi must be defeated, so he still can do one good thing for the Uchiha clan. Sasuke has now surpassed the point Itachi was unable to surpass, but he will later in the manga realize that revenge alone won't let him achieve his goal. The main theme of the story is friendship, and Sasuke will grow stronger with ambition to protect Naruto from Madara since Kyuubi is originally his target. Their final encounter with Madara will be Sasuke and Naruto fighting together, side by side, for the sake of the story's underlying message.
Karin will notice Tobi's chakra and that will be the team's lead towards the Hidden Rain where Tobi and Pain are waiting for their arrival. As they enter, Pain will soon notice their presence with his rain. He will appear before them and defeat them all in battle. But as Hanzou did with the Sannin, Pain will honor their strength and tell Sasuke that his eyes are needed in Akatsuki. Suigetsu (who defeated Kisame outside the Uchiha hideout), Karin and Juugo will also get a spot. Pain has realized their strength and offers Sasuke the mission to claim the 8-tails in order to achieve more powers (though his real reason is the Sharingan which also Tobi understands will surpass Madara's). Sasuke has theories about Tobi influencing Akatsuki and accepts Pain's offer (to his comrades' surprises) in order to get closer to Tobi and at the same time get more power in order to surpass even Madara himself. Team Hebi is against the invitation at first, but follows Sasuke's words. They all have reasons to stay close to him.
Naruto will later encounter Sasuke in an Akatsuki robe, leading to new intrigues within the storyline. Enraged, Naruto will charge at Sasuke and a minor fight will occur, which will be stopped by Juugo (Sasuke's Akatsuki comrade because Sasuke is needed to keep him in control [Suigetsu and Karin will be paired for the sake of comic relief]) and the Konoha team.