Why ALICE did it... (spoilers, of course)
Okay, obviously this is just my opinion, and yes I've read the article where the writer says that the story "transcends guilt or innocence".
Personally, I believe that Alice did it.
Why?
First of all, we know that only the guilty sleep, right? That's interrogation-scene rule #1. (My father in law, a retired cop, says that it is actually very very true to life.) Alice sleeps in the interrogation room. Darlene does not sleep while in the cell.
But let's back up. In the hotel, when they are getting into their cab (I know, a lot of people have the cab-trunk-switcheroo theory), Alice gives an uncomfortable look back toward the trunk when she is no longer holding the bag. Of course, the obvious questions are how could she not notice all that extra weight & what was she doing in the hotel room for so long while she claimed to be "washing her hair"?
But, I think more than anything, if you look at the movie very mechanically, the fact that she sleeps in the interrogation room is THE dead giveaway. I mean, that is WHY they show that in movies. Its foreshadowing. If you know anything about crime movies, you know that sleeping means you are guilty.
Next time you watch Law & Order or something, see if I'm wrong about the sleeping thing. :)
But anyway, I think that, although the writers didn't want to directly implicate one character or another, they placed a little 'tell' in there for those of us who just HAVE to know.