The current ending: Every version of the climax that I've heard about has Micah dashing downstairs after Katie is dragged out of the bed/room, where some sort of off-screen ruckus takes place and only a bloody Katie returns. In this version, Katie tosses Micah's body at the camera from the hallway (a great money shot, I'll admit, albeit one given away in the trailer), kneels down beside his body to either sniff or lick it, and then approaches the camera before smiling and then lunging at the lens. A title card informs us that Micah's body was found on whatever date, and that Katie's whereabouts are still unknown...

A little too Hollywood cliffhanger-y for my tastes, but it plays better on the screen than I'd heard.

The original ending: This time, Micah stays downstairs as Katie returns to the bedroom, covered in blood and now holding a knife. She proceeds to rock on the floor next to the bed for something like two days straight.* We hear a friend (the girl who offered that Katie stay at her place, I'm guessing) leave a message on their answering machine, wondering where they are. She later stops by, presumably finds Micah dead downstairs, screams and leaves. The police then show up later, with Katie only awakening from her catatonic state as they come up the stairs, with guns drawn. She's covered in blood, holding a knife and very confused as to what's going on, and so the officers proceed to fire on her. The title card then dedicates the film to the couple's memory.

This one is a bit anti-climactic, yes, but I think it's a bit more tragic that Katie's been tormented all her life, only to face an unwitting death such as this.

The alternate ending(s): Now, several endings were shot after the film was picked up. The only one of those that I've seen described anywhere was screened maybe once at a test screening, and Peli suggests that we may see it yet on the DVD. In this version, Micah dies downstairs, bloody Katie returns to the bedroom with knife in hand, and she proceeds to slit her own throat in front of the camera (without looking right into it).

I personally prefer this one because Micah makes a gesture with a knife in the kitchen very early on, suggesting that whatever the problem is, they'll [slicing motion] take care of it. That might've made for a nice bookend, and would've been disturbing regardless.