I also have the same problem - cracks on macbook white unibody (late 2009 version). One is where power cable is plugged in and two are on the back part of the screen.
I tried the service, but they charged me like 100 USD for just checking and concluding that crack (at that time it was only one, near the power cable connection port) wasnt apple's responsibility. But no real explanation was given how it could have appeared. They said maybe i havent used it carefully enough.. Well i use my computer a lot, it's the main tool for my design studies, i carry it around a lot and so on (isnt that the purpose of laptop?). But i know for sure it has never been dropped and always is carried around in a protective case.. So i wont admit my fault.. especially after noticing last cracks.. this is too much now..
other than that i love apple.. but macbook white as a tool for a student in terms of built quality has let me down..