While I'm far more interested in this film than just about anything else currently in development (Dredd being a close-ish second) I can't help but wonder how Del Toro intends to tackle the actual narrative of the film. The book as it stands is basically a summarised history of the Elder Things with a thin mystery plot as a framing device. For most of the film, all the characters really do is wander though an abandoned city, reading dusty old murals before hearing a funny noise in the shadows and run away screaming like little girls. It's great as a book, but there's no way it'd work as a film "as is." Indeed this is a problem with a lot of the various attempted adaptations of Lovecraft's work over the years. I say attempted because none have really hit the mark and only one or two (understandably) made any effort to stay true to the source material.
Actually to date the best Lovecraft adaptation I've seen is not a film at all but a video game; Dark Corners of the Earth
. Instead of picking one story and padding it out or poaching ideas and making up somethin totally new, the developers made the interesting choice to seamlessly merge 'Shadow over Innsmouth' with 'Shadow out of Time' and it works surprisingly well. So I wonder if GMT might do something similar with 'At the Mountains of Madness'; perhaps merging or somehow tying it in with 'Call of Cthulhu' since the stories are peripherally and contextually related.