Though there were earlier rumors of Arnold Schwarzenegger, who may have been in talks, James Cameron today confirmed that despite the fact that Miles Quaritch met a fatal end in the first film...he'll be back. As long as it's not too preposterous a twist that does this, I'd say that's pretty good news - anecdotally, it seems like even people who were lukewarm on the movie enjoyed seeing this veteran character actor have his day.
[I asked Latino review's El Mayimbe, who had the Ahnuld rumor first, on Twitter if this news means Schwarzenegger's out. He responded with an emphatic "No!"]
My guess - it's been established that the planet can preserve consciousness after death, but our assumption, I think was that all such consciousness preserved would be happy and harmonious and hippie-dippy. What if it's not that simple? (If it's going to last three more movies, it had better not be.) Either Quaritch is such a badass that he can overwhelm the Eywa forces and control parts of the planet himself as a disembodied spirit, or Eywa is not the simple essence of good that was implied.
Either way, it'll be fun to have the most entertaining character back.