I agree fully with Chris. This is based on a survey of about 50 photographs of landing B777s showing the wing undersides. I can find only one picture possibly suggesting wing surface frost in the tank area (ANA-Star Alliance plane).
I would imagine the airflow in most cases prevents frost (or ice) to form until the plane has landed and slows down. The wing underside is also a high pressure area, meaning that moisture is less likely to condensate on that part of the wing than on a low-pressure area.
That said, most of the pictures are taken in conditions of less humidity than the weather BA38 had.
But if it's frost that is visible on the BA38 picture, it might be a fairly unique occurrence?
Here are some pictures:
http://www.airliners.net/photo/Korea...-ER/1341078/L/
http://www.airliners.net/photo/Briti...-ER/1339905/L/
http://www.airliners.net/photo/Air-C...-LR/1342818/L/
http://www.airliners.net/photo/Conti...-ER/1339702/L/
http://www.airliners.net/photo/Jet-A...-ER/1338232/L/
http://www.airliners.net/photo/Star-...-ER/1336739/L/
http://www.airliners.net/photo/Briti...-ER/1333455/L/
http://www.airliners.net/photo/Briti...-ER/1326063/L/ G-YMMM
http://www.airliners.net/photo/Korea...-ER/1325100/L/
http://www.airliners.net/photo/Korea...-ER/1321846/L/
EDIT: Based on some more searching, frost on the underside is not totally uncommon but still fairly rare. A search turned up a total of 3 pictures with visible frost under the tanks.
http://www.airliners.net/photo/Finna...-11/0991454/L/
http://www.airliners.net/photo/Virgi...443/0764483/L/
http://www.airliners.net/photo/Gulf-...101/0701662/L/