Running in the rain. Phone worked fine when I got home, but I didn't take it out of its case to dry.
No backlight when I got up the next morning.
Followed the "dry this, scrub that" directions to no avail. When I swapped the display assembly from a working 3GS into the broken backlight 3GS, it was obvious that the display assembly was the broken part: the bad display didn't work in the good phone, but the good display did work in the bad phone.
iFixit's $99 display assembly is a bargain compared to buying a new phone. All good.
I didn't pay attention to all the directions, so I tried at first to pry the display assembly off the phone. THAT IS A BAD IDEA.
Firstly, the chrome surround piece is not part of the display assembly. It doesn't need to come off.
Second, you really do need a suction cup to pop the display free, but any of the cheap shower things (soap dish, mirror, toothbrush holder) with a suction cup wall attachment will work just fine.