It seems that a small number of consoles are incompatible with the next dashboard update and will stop reading discs once updated. If this happens to you, MS will give you a 250GB Xbox 360 S and 12 months of Xbox Live for free. In fact, they can even detect over Xbox Live which consoles are affected and they're sending emails out to customers already.
Source: -
BREAKING: The Latest XBox LIVE Update May Affect DVD Drives | Kotaku Australia
Source: -
BREAKING: The Latest XBox LIVE Update May Affect DVD Drives | Kotaku Australia