I think someone said this in the thread, but I'm not sure. And that is the fact that Qui-Gon, unlike all other Jedi who decease, did not disappear after death, we saw his body burning. My prediction to this is that only Jedi who disappear after death and take spiritual form return as "spirits", albeit the only one we truly see is Obi-wan undergo this but if anyone here is familiar with EU novels it goes for literature as well. Therefore, my conclusion to this is that Qui-Gon can NOT take spiritual form because his body did not disappear. Why didn't it disappear? Simple. His defiance of the Jedi Council kept him from being "one with the Force".
This doesn't mean that he will definitely not be in Ep. II. I love pondering the fact of ghosts vs. spirits. Ghosts are notorious for remaining because of an "unfinished task". Could this be that Qui-Gon might return as a ghost [rather than spirit] to try and obtain a status of being one with the force? Therefore he can take spirit form.
There is just one problem, what would be so significant about spirits rather than ghosts? Does a spiritual existance hold a significant well-being rather than a ghost? I don't want to indulge in a discussion of nirvana, but maybe someone can help me with this.
A point I would like to make is I fear GL's focus of maintaining a well-balanced plot. If he does have Qui-Gon return as a spirit (rather than ghost, haha) then it will only confuse any Star Wars fan that has ever understood the movies, because we know through ANH's plot that Jedi who disappear become spirits. (Whether or not they are one through the Dark or Light Side). Hence why Yoda, Anakin, AND Obi-wan all became spirits.
So, is there anyone who would like to improve my ghosts vs. spirits theory. I am open to questions, comments, and sarcasm.