As a couple of readers here and over at my Naked Bible blog have brought up the recent proposal that the Coptic text in question is a fake, I thought I would direct readers to this short (6 pp.) explanation from Prof. Francis Watson as to how he thinks it was done. Even if you don’t read any Coptic I think you’ll be able to follow it. Thanks to my readers and Mark Goodacre’s NT blog for the link!
My take (as noted in the comments) is that the explanation is coherent, but needs to be bolstered by C-14 testing. However, that might not do any good (cf. my references to the old Irving Wallace novel, The Word). The physical features could be authentic and yet faked (in the Wallace novel the ink was hand-made from materials that would pass C-14 testing and the parchment was cut from a genuine uncial — some of them have blank pages).