THE STORY OF THE SECRET GOSPEL OF MARK is told by Charles Hedrick in a Biblical Archaeology Review article ("An Amazing Discovery"). Read it all, but most interesting to me was his account of the fate of the manuscript after Morton Smith's publications on it came out:

In light of the issues raised by Smith’s two books, efforts were made to see and examine the Voss book containing the letter of Clement, which Smith claimed to have found in the Mar Saba library. In 1976, three years after the publication of Smith’s books on Secret Mark, three Hebrew University scholars (David Flusser, Shlomo Pines and Guy G. Stroumsa, then a graduate student at the Hebrew University), in the company of an official of the Greek Orthodox Church (Archimandrite Meliton), went to Mar Saba and managed to “relocate” the book, after some searching around in the tower library where Smith left it. Because of its significance, all concurred that it should be taken to Jerusalem and secured in the Patriarchate library. They hoped that a scientific test of the ink would demonstrate the date of the inscription of Clement’s letter, but such a test was not permitted. Subsequent scholars who visited the library later were not permitted to see the book.
In 1980 Thomas Talley, a professor at General Theological Seminary in New York, reported in an article that he was not allowed to see the letter because it had been removed from the book and was being “repaired.”13 Shortly after the book was deposited in the Patriarchate library in Jerusalem, the librarian (Kallistos Dourvas) removed the two folios containing the Clement letter in order to photograph it. He then replaced the two loose folios at the back of the book.14 And then the Voss book itself was “misplaced” in the library and could not be located. In June 2000, however, the Voss volume was relocated in the Patriarchate library, but the two folios containing the letter of Clement were missing. They are still missing.

So apparently a number of people besides Smith have seen the pages in question, but they are now lost. Until they are recovered, I suspect the debate on their authenticity (not to mention the authenticity of Secret Mark) will continue.

Other articles in the same issue of BAR are available only to paid subscribers. April De Conick comments here. No comments yet from Stephen Carlson, but congratulations to him anyway on the recent sixth anniversary of his blog Hypotyposes.

UPDATE (5 November): For Morton Smith's translation of the text of the letter, published by BAR, go here.