Don't really want to add to much to this. John Piper certainly doesn't equate Old Testament and New Testament prophecy and I concur completely. I have based most of my perspective on Piper's and admit that it is marginally just a bunch of definition changing.
However, I appreciate the full extent of Piper's perspective and his willingness to push the bounds of people's comfortableness with the subject. The acknowledgment that the gift is still functioning in the church is incredibly crucial, in my opinion, to the continuing exegesis and preaching of the Scripture. Thank goodness the Lord doesn't require us to believe in the gift of prophecy to impart it in us and use it to teach His people.