I can't speak for everybody, but I knew this, which is why I bought a black one (that and because I didn't want white, but I digress). I learned of this back when NSider still existed. The composition of the plastic that they use for the white DSL is too brittle. Never mind what Ninty said about it (something about some screw being too tight), it's the composition of the white plastic that is at fault.
I don't know if it's ONLY the white one with the issue, but I've never heard of any other colors having the problem myself. Pics of the crack were ALWAYS a white DSL. I've had my black DS since they were released in USA and it's still fine.
How long have you had the DSL? If it's less than a year, Ninty should fix it for free.