A new beta version [build 12606] of the Fall 2010 [alias 'Kinect'] dashboard has been leaked on the internet a few days ago and it seems like Microsoft added new protections to it. Several users seems to report problems booting certain games from recordable media on 360s with a modified DVD FW.
One of the titles affected seems to be Fable III. The game boots fine from any media on the current dashboard on a 360 with modified FW, but fails to boot from recordable media on the leaked/12606 beta dashboard ["This disc is unreadable"] - while the retail disc will work just fine according to #FW@EFnet.
Right now it's unclear which games, DVD models and/or modified FWs are exactly affected.
we suggest you do NOT update if you have a modified FW until we know more about these issues.
JTAG users should of course not update at all (will lock-out old CB and make the JTAG hack impossible), they should wait for a new rebooter.