This started as a response post, but I think it merits a page. Inception doesn't overtly explain how Cobb gets from "his limbo" to Saito's. It also doesn't explain notable differences between the two limbos (why is Fischer young and Saito old?) or why Cobb appears so disoriented in the latter but not the former. Understanding these differences is, in my view, the key to understanding the movie Inception.
As the film tells us, Limbo is unconstructed dream space. It's dangerous to go there because it's easy to forget that you're dreaming--in other words, in Limbo you get disoriented. It doesn't operate like other dreams in that (a) there are no projections--only what you build, and (b) the dream won't collapse, at least not for decades. To get out of Limbo you have to remember that you are dreaming.
This is what the film tells us. It reinforces this through the story of Mal and Cobb--how they ended up in Limbo, and what it took to get out.
Given this, we should question whether what Cobb and Ariadne experience together is actually Limbo. I don't think it is. I think it is Cobb's dream of Limbo from when he was previously there with Mal. He believes it is Limbo--much like Fischer believes that dream level 1 is reality--but it is not. In this dream level 4, Mal is a projection, as always, and Fischer is also a projection. That's how Cobb knows where Mal and Fischer will be. In other words, on this level Cobb and Ariadne are sharing a dream that appears to be the Limbo that Cobb remembers, but is not.
How do we know this? There are at least two clues. (1) Most importantly, this level is unstable. Buildings are collapsing into the ocean, the houses are dilapidated, and towards the end there is a huge storm and everything is falling apart. How could this possibly happen in Limbo, which is unconstructed dream space? It couldn't--everthing is collapsing because it is a dream. (2) Wouldn't it ruin Cobb's inception of Fischer to have Fischer bound and gagged by Cobb's dead wife? I think so. On this level Fischer is a projection of Cobb's, much like Browning is a projection of Fischer's on level 2. Note that Fischer is revived on level 3 by a defibrillator, so he does not need a kick on any deeper level necessarily.
The real Limbo is what Saito and Cobb experience. Saito is there because he died on level 3 from the gunshot on level 1. Cobb is there because he drowned on level 1 in the back of the van. Notice how disoriented Cobb and Saito are here. Cobb doesn't appear to know where he is as he eats his oatmeal. It is only through the shared memory of those lines of dialogue that Saito and Cobb are able to remember that they are dreaming and get themselves out of Limbo.
Why is this distinction important? Because there is a secret inception--Cobb's. Cobb thinks that he is in Limbo, he thinks Fischer is dead, and he thinks that his regret over what happened to Mal is what led to these catastrophes--as Ariadne says, Cobb's regret and guilt is what feeds and creates his projection of Mal. This realization leads him down further, and it is at level 4 that he realizes that he has to let go. He realizes that he fulfilled his marital committment to Mal and that he did what he could for her and has to let go. (Whether this is true or not, we don't know. It's possible that Cobb and Mal actually didn't "grow old together" in limbo--it could be merely a positive memory that Cobb convinces himself of, much like Fischer became convinced that his father was "disappointed" that he tried to emulate him, which wasn't actually true.) This is a positive idea planted deep into his mind that he thinks is his idea. This idea leads Cobb to make radical changes in his life--he stops worrying about what is real and what is not (recall that he does not wait to see if the top drops) and lives without regret by returning to his kids. (Whether the ending is a dream or not is another question entirely.)
But is it really his idea? If you watch the movie again, you should closely watch how Saito and Ariadne act towards Cobb. You'll notice repeat lines of dialogue and Ariadne's persistent fascination and interest in Cobb's thoughts. In my opinion, Saito and Ariadne were involved in an inception of Cobb. This inception might start in the real world--if what we are led to believe is reality actually is reality--but it must be masked by the mission to incept Fischer, or else Cobb won't venture further. Notice that Saito and Ariadne insist that they be brought along on Cobb's mission--Cobb didn't intend to take either originally. Notice how Saito repeatedly tells Cobb that he doesn't want to be "an old man filled with regret, waiting to die alone." Notice the music that is used to signal the collapses of dreams means "regret nothing." Notice that it is Ariadne that convinces Cobb to enter the last dream level, and notice that Ariadne brought along the dream equipment in her backpack.
Whether the rest of the team was involved is unclear. Whether the Fischer inception was a ruse or real is also unclear--I lean towards real, but who knows. But the key to understanding the film is that Cobb is an untrustworthy narrator, just like Guy Pearce in Memento, and that he is actually being incepted during the film. Understand that, and you'll get much more from the movie.
Taking it a step further, I think that Cobb actually was not under suspicion for his wife's death. He merely convinced himself that he was so that he could avoid going back to his kids, to avoid experiencing the tremendous guilt that he feels. This is why he does not look at their faces--to avoid the guilt from having caused their mother's death. Michael Caine is Mal's father, and his scene early in the movie jives with this view. "Come back to reality, Dom." "A stuffed animal won't convince the kids that they still have a father."
Caine sets up Cobb with Ariadne, so it's entirely possible that he instructed her to perform an inception down below. This would mean that Saito would have to be in on it too because he agrees to make the phone call.