As a thought it's awesome, but making it would be a different story. Of course it just couldn't have the same feeling as the game does, since you can't play it yourself and decide what to do and where to go. In my opinion it's nearly impossible - if not completely - to create exactly similar horror in games and movies.
But maybe the movie could be more about the characters, like Daniel and Herbert's expedition in Algeria, Daniel's attempts to seek help, meeting Alexander etc. There was lots of scary stuff happening even before the events of Amnesia. The movie wouldn't need to be so sneaking and exploring oriented as the game is. Like the game, the movie could also happen in the "present" but while Daniel's reading notes about the past we'd see them all actually happening. There could be some gatherer encounters but not so much silent time while wandering around.
And then there's this really annoying thing that irritates me every time I see a movie that's based on a book or a game:
rushing and cutting too much. I understand there's a limited time for a movie but it still feels bad that they have to cut off so much important stuff. It's terrible to watch a movie where's no any breathing space or getting to know the characters, but just rushing to one important key scene to another. I don't want that to happen to Amnesia. Maybe a short tv-series or a mini-series could work better than a movie.
Anyhows, it'd be too cool to see Daniel and pals on the big screen. Then I could just die away.