yep, this happens alot of the 737 series, when temperatures are below freezing that is.
Normally ice starts to form under the windows which are directly above the wing,not exactly sure why this happens, but one thinks its something to do with smaller gaps between the outer and inner parts of the fuselarge.
An engineer did say to me it was caused by some parts in the wing framework being exposed to extreme temperatures, which can directly effect parts of the inner fuselarge due to the close proximity of these members to the inner walls.
G-STAW