Sadly enough this same thing happened to me. Pages 1-12 of my DMG fell apart on about day 2. I returned it to the book store and they replaced it for me, but the manager of the store (just happened to be the one that helped me, i did not put up that big of a stink) and i went through the 4 they had on display and two of them were obviously in bad shape right out of the case. the first couple pages of the book were not bound properly, but it was only in the DMGs. I took one of the ones that looked good and he defected out the one i returned and the two we found.
It seems like might have been like, one batch of them were not bound well enough, because the other books are fine and my house mates DMGs are fine. Sucks for WOTC if a lot of them are having this problem though.