A new update for the 360 firmware has an issue with a very small amount of XBoxes using a specific kind of hardware, rendering it unable to read disks.
Microsoft had said “Following a recent update to our system software, we have become aware of an issue that is preventing a very small number of Xbox 360 owners from playing retail game discs.”, the people who will be affected by this are in the minority however. Just a note, this is not related to their public beta testing of some firmware.
However, Microsoft isn’t going to break your Xbox and laugh all the way to the bank, and have stated that consoles broken by the update will be entitled to an automatic replacement of a 360 with a 250GB hard drive. It is rather irresponsible for them to make this patch break a minority of consoles, but catching it before release and telling people they’ll replace broken consoles. They can detect which consoles will be affected via XBox Live and are sending out emails to those with affected consoles.