Brent Nongbri, "The Use and Abuse of P52: Papyrological Pitfalls in the Dating of the Fourth Gospel," HTR 98.1 (2005): 23-48
Ken quotes Nongbri's conclusion:
What emerges from this survey is nothing surprising to papyrologists: paleography is not the most effective method for dating texts, particularly those written in a literary hand. Roberts himself noted this point in his edition of P52. The real problem is the way scholars of the New Testament have used and abused papyrological evidence. I have not radically revised Roberts's work. I have not provided any third-century documentary papyri that are absolute "dead ringers" for the handwriting of P52, and even had I done so, that would not force us to date P52 at some exact point in the third century. Paleographic evidence does not work that way. What I have done is to show that any serious consideration of the window of possible dates for P52 must include dates in the later second and early third centuries. Thus, P52 cannot be used as evidence to silence other debates about the existence (or non-existence) of the Gospel of John in the first half of the second century. Only a papyrus containing an explicit date or one found in a clear archaeological stratigraphic context could do the work scholars want P52 to do. As it stands now, the papyrological evidence should take a second place to other forms of evidence in addressing debates about the dating of the Fourth Gospel.It looks like an important article -- it's a staple of introductory courses on the New Testament that P52 provides us with a nice fix-able date in the very early second century that acts as an anchor for a lot of other datings. It looks like people will need to revise those sorts of over-simplistic appeal to over precise attempts to date this fragment.
Update (Thursday, 14.28): Sean the Baptist comments.
Update (Sunday, 1.04): Stephen Carlson has some useful comments on Hypotyposeis.