A couple of random thoughts about any attempt to make this series into a movie:
What about Amy Acker as Vin? Same 'pedigree' as Summer but I think she has more range, at least that she's demonstrated on screen... I agree with some folks that Summer would probably work great for Vin in Book 1, but Acker feels like she could pull off the more serious stuff in Books 2 and 3 a bit better. If we go outside the Joss Whedon group for Vin, I'd be tempted to suggest Mia Kirshner.
It's all fun to speculate but I think if Mistborn ever got made into movies, there would have to be some fairly serious deviations from the books in two parts:
1. The parallel 'stories' at the start of each chapter. I have a hard time seeing how these could be included in any fashion. This takes a great deal of the uniqueness out of the books if they can't be included. Additionally, they're almost necessary to make "The Well of Ascension" work because of Ruin's ability to change the text. And of course, the log of Rashek is key in the first book. There will have to be a way to get the same information across without a long period of a character just reading the log/prohpecies in one chunk.
2. A LOT goes on in the minds of the characters. How do you include the 'visitations' by Kelsier, Reen, 'God' to the appropriate characters correctly? How do you portray Sazed's struggle with religions that's so fundamental to the third book? How do you portray Vin and Elend's struggles to define themselves? These are all things that are at least as important, if not more so, than the dialogue and action that would be much more easily translated.