An upcoming 360 firmware update is set to introduce a new disc format to the platform, but will prevent some consoles from reading discs at all.
Apparently a verified email sent from Xbox Live customer support to one beta tester acknowledged the update prevents “a very small number of Xbox 360 owners from playing retail discs”.
Said update it being introduced between the 19th and 30th of May. It reportedly re-flashes the DVD drive to a new stock firmware and adds support for a new XGD3 disk format that allows for an extra GB of usable storage on game discs.
It seems it was the 60GB Xbox 360 Pro from 2009 that was effected in the incident, so if you have one of these machines then watch out. Microsoft have said that they will replace any consoles negatively affected by the update.
Thanks to Kotaku for the info.