Update: I carefully compared this bird to this one from Galveston in November - I believe that it is the same bird, now looking even less thayeri-like than six weeks earlier.
This gull was photographedain the Coastal Plain near Houston and Galveston, Texas on December 28, 2005; it was one of two almost identical birds seen there. It seems to match very well with some of the lighter Slaty-backed Gulls shown on Asian gull websites - but are all those birds pure SBGUs?, and is there a plausible New World hybrid combination that would account for this (these) bird(s)?:
In the image below, it is on the left, with a smithsonianus HERG center-right, and a 2nd-winter LBBG on the left:
