Cameron tells the LA Times that he doesn't think the two actors turning down the role had anything to do with the atypical working conditions, rather that he wasn't too enthusiastic about recruiting them for a trip to Pandora. "Honestly, did I go out and try to woo them? No. I had my heart set on Sam. Maybe they sensed my lack of 100% commitment from me. Maybe it was the subject matter. This was a big 'Star Wars'-type movie. They're both serious actors."
Now if that reads as though some of the higher ups at Twentieth Century Fox asked Cameron to at least go out and talk to some bigger, A-list actors to spearhead their considerable investment, that's because it's exactly what it was talk is precisely all Cameron did. Personally I can't imagine either Damon or Gyllenhaal wearing the digital loincloth of the Na'vi, which I suppose is a testament to how much Worthington made the role his (or the fact that I saw Avatar three-too-many times in theaters).
What say you? Would you have liked Jason Bourne or Donnie Darko to tame Toruk Makto? Or a third party, for that matter?