Given that the Gershwin estate has (1) allowed (or even suggested, as I've read elsewhere) some of those controversial changes in the book and score for the current production (some of most radical of which have been mercifully returned to "normal" by the producers), and (2) that they have even "officially" changed the work's title to the absurd and laughable "The Gershwins' Porgy and Bess"... I would hope that they are no longer withholding the film on the basis of any real or perceived objections to, ahem, artistic standards.
Another obstacle to its release, if I've heard this correctly over the years, has been Sidney Poitier's longstanding objection to the film.
A collector's print shown at the Ziegfeld a few years ago was either very problematic in itself, or the folks at the theater just didn't know how to handle it. If I recall correctly, we were told by the author of the then-new Otto Preminger biography that it was one of the few remaining elements and that something would have to be done soon if it's to be salvaged for a proper home video release and for posterity.
The film itself was gorgeous. I'd seen it once as a kid (it was single-handedly responsible for my lifelong love of Gershwin's music, exactly as the film of "West Side Story" was for the music of Bernstein just a few years later). This time, in 2008, it was obvious to me how incredibly static the staging and cinematography really were...almost shockingly so. There are musical and dramatic moments that would have been highly served by a close-up or some kind of movement or cutting. Nevertheless, it's a masterpiece in its own right and a priceless document. It retains more of the opera score than you'd expect, and the talent involved is beyond incredible.
Edited by Chas in CT - 11/18/11 at 7:22am