Both James McAvoy and Tom Cruise have other projects lined up at the moment, with James McAvoy filming X-Men: First Class and Tom Cruise shooting Mission: Impossible in October. However, At the Mountains of Madness isn't expected to start filming for another year, so it's possible that either actor could sign onto the film.
Here's a synopsis for the novella below:
"The story is written in first-person perspective by the geologist William Dyer, a professor from Miskatonic University. He writes to disclose unknown and closely kept secrets in the hope that he can deter a planned and much publicized scientific expedition to Antarctica. On a previous expedition there, a party of scholars from Miskatonic University, led by Dyer, discovered fantastic and horrific ruins and a dangerous secret beyond a range of mountains taller than the Himalayas."
We /guillermo-del-toro-and-james-cameron-are-at-the-mountains-of-madness/reported back in July that James Cameron was producing the movie and that it will be shot in 3D. We'll be sure to keep you posted with any further details regarding At the Mountains of Madness as soon as we have more information.