I just started reading Thomas and Tatian: The Relationship Between the Gospel of Thomas and the Diatessaron by Nicholas Perrin. At the moment, I am not sure if I agree with his case, however, it has to be said that I haven't found any of the excessively early arguments (e.g. Koester) persuasive in the slightest. My first thoughts are that he is pushing it a bit with the late date, however, there are many questions that have been raised by Gilles Quispel between the Diatessaron and Gospel of Thomas which need to be taken seriously. And as Perrin has rightly noted, scholars really haven't.
Anyway, why I am interested in the question has to do with my re-evaluation of Bauer. Thomas represents one of the earliest so-called Gnostic texts in Egypt - and depending on the original version, or in this case a Greek fragment, the model has to be adjusted. Do we go with the idea that the early Thomas contained an early proto-orthodox tradition -some independent of the synoptics, which were later Gnosticied? Such would account for the simple fact that some of the common sayings demonstrate a clear theological gnosticising tendency. Do we acept the text as simply another mid-second century and beyond Gnostic text?