What is the nature of New Testament Prophecy?

Tonight I am listening to a sermon by John Piper entitled Using Our Gifts in Proportion to Our Faith, Part One (mp3) which has been quite provocative in my understanding of what prophecy is in the New Testament. Piper is going through Gaffin and Grudem's differing notions of what prophecy in the New Testament is. The greatest difference he has mentioned is that Gaffin wants to see it as always infallible, while Grudem wants to affirm it's fallibility, that is, it's need to be discerned to be from God and not from man, although I'm not sure if he will say that all prophecy that comes true is necessarily from God. Anyway, these questions are going through my mind and it's the first real sermon I've heard Piper give on this issue. Perhaps it will be good for me to consider the wisdom of the traditions as they attempt to make the best sense of the New Testament, because the way you make sense of it there bears great consequence upon how you understand its role today. I'm interested in these "gifts" of "prophecy" and especially interested in their eschatological significance whatever that may be.