That's where Nolan let's you decide the ending. More like you interpret the ending.

...dont read too much into the where did the briefcase with the apparatus go, if you wanted to take it at face value, remember they had the female stewardes who assisted, & like the little japanese boy earlier (even if it was dream sequence).

add 'what if' to the tip of your tongues at the ending but in my case, cobb's & probably saito, didnt come out of 'limbo' in the end. (maybe more of cobb then saito since we are taken to cobbs wish ending)

cobb wanted & got his "wish" of finally concluded his vovage in reuniting with his children but its because he choose that state of dream. **(see bottom of my spoiler)

several key elements that gave away as one TS pointed, his children in the end, they never seem to grow up, as is usually we have of pleasant memories of, things dont change or aged much in our dreams....&...&..
...the children's clothes. remember how they looked in his memory & in the end??
(here is some trivia for those going for second screening )   
alternate ending [maybe]
which we wont like to see but as hollywood execs might have an insurance on   
well, we see cobb's body in plane, maybe paramedics attending to him cause he is in a comatose state, & arthur, yusof & rest of gang got up but in a frantic state, while Robert Fischer walks out of plane a little bewildered (but they had already successfully incepted the idea to him)
in the end don cobb got his final wish, & executing his final job to perfection.
then music score 'paradox' the comes on     

  ...  528491
 
warning, i warn you 
http://528491.com/
there are several references of this numbers in the movie, the room no, the floor they were on etc

yes nolan had some little references of these number played in the movie, to have the cult fans pointing each & every time its crops up. the 52's, 491.. etc
* i'm with the same agreement as the quote below of both saito & cobb's ultimate fate in the end

you know when everyone got 'kick'-ed except saito and cobb, how did saito and cobb got kick after cobb made saito realised it was all a dream? i thought they had to synchronize the kicks?? unless he could get kick from limbo then to level 4 to level 3 and so on. plus...i forgot why did they wanted to sychronize all the kicks?? can someone enlighten me?

Ahh yes thanks for reminding me, that was one of the questions I had in mind while watching. How does one get back to reality from a limbo state? In fact, can someone tell me the whole story about Cobb and Mal? They were experimenting with dream states, took a high dosage of sedatives, lived a long time in their dream world, decided it was enough, killed themselves by being run over by a train, wound up in limbo, got old there, and then somehow getting back to reality? So how did they get back to reality? I'm not sure if I got their story correctly. Shit I cant remember anymore lol.

They still look young when they committed suicide; getting run over by train; and went back to reality but during the last part, there was a scene showing that they grew older;  or is that just what Mal is thinking. Or did they gone back to limbo after getting back to reality.

Err... Somehow I didn't catch that part of what you are saying. What's the thing Cobb incepted?

Yeah that's what I was talking about, which leads me to think now that suicide took them from their dream to limbo, not from limbo to reality. Because they were still young. At first I thought that suicide was the one that brought them back to reality, but I think, later in the film, they showed them getting old in their dream world. So yeah.

So when Mal jumped off the building, they're still in dream state?    

"That this world is not real."

So when Mal jumped off the building, they're still in dream state?   

Yeah, that what makes me confuse also; coz after the scene with train suicide, they woke up in their house. And I thought they went back to reality; but Mal can't accept that part of reality and want to go back to dream state. And continued with the scene with her jumped off the building. I'm not sure with this continuity which part are they in; reality or dream state.

Damn!!! He even planned it for marketing purposes!!! It also spells KATHY. through the use of your phone with T9 on.

528491