There has been some interest in creating such a standard, but at the present time there is no one pushing for this.

In my experience, to create such a standard requires some individual to push the goal forward, first by getting a sufficient group of people together (e-mail list or discussion forum), then by putting out a strawman proposal that can serve as a basis for development, and then by trying to reach consensus. What the DWI/DTI community is lacking at this time is someone willing to take on this catalyzing role -- someone whose credentials in this area are solid enough so that other luminaries will listen to him. Not necessarily defer to him, but listen enough to start the process. And then keep listening as he pushes the process along.

If I were to say anything, it would be that the NiFTI-1 extension should be in an XML format, to facilitate cross-platform communications.