Based on my opinion and understanding of the show after watching it twice.

There are 2 ways to enter limbo.
1) By dying in a heavily sedated dream
2) By diving in too deep.

Cobb and Mal first went to limbo by diving in too deep. It was clearly stated that they got into limbo while experimenting with dreams within dreams.

Take note that the sedative is only stable up to the 3rd level. It was never formulated for a 4th level.

Another thing you'll notice is when they were talking about limbo the first time, it was said that it's just a raw collection of memories in which case would be his"cobb". And true enough, limbo belonged to cobb probably because he built it the last time he was there.

Saito and Fischer went into limbo by dying while Cobb and Ariadne went there by diving in.

I also disagree with the theory Mal was right it's a dream all along.
Why? Because she never appears in the real world. Ever.
Hence all of this line up to say it's Limbo and not level 4.

As for the end, I would like to go with the reality theory as well.
Because in the end, all of them will wake up.
Saito was already old in the final cut scene and is still remembering the agreement.
They both know they're in limbo and will obviously make the right choice.

Their bodies are all dead in level 1,2 and 3 which means killing themselves in limbo would immediately wake them up.

Hence it all lines up it's reality.
Yeah, his children are wearing the same clothes but the totem was wobbling as well. But I believe this a trick Nolan used on us to get repetitive viewings.

This post has been edited by LegendLee: Aug 6 2010, 01:58 PM