I've got a DSi for little more than a year,
and the R button is completely broken,
the top half is somewhat 'moveable',
and I've, sadly, also experienced my L button not working, lately.
I treat my DSi the best I can. I've been extremely careful with it since I got it.
It never fell, I never shake it, I never hit it or anything...
Believe me when I say that I know no one who treats his stuff better than I do.
My DSLite also had a cracked hinge, but that could be because my 'somewhat' more chaotic brother played on it sometimes.
When I still had a DSLite, I always thought it was the people's fault for not treating their consoles right.
Now, however, I know that it can absolutely happen to anyone, and that these problems are sheer coincidence. Ofcourse, trying to eat your DS will help breaking it, too, but DSes are just really random.
Some break, some don't. I've even seen the DSLite of my neighbour break,
but he always takes it to restaurants etc. (yeah, it's weird) and never takes good care of his stuff,
because 'his parents will always buy him new ones'.
I sincerely hope that Nintendo will make the 3DS a lot sturdier - or at least gives us better L & R Buttons.