Microsoft seems to have added a new protection into the beta build 12606 of the Fall 2010 alias 'Kinect' dashboard leaked on the internet. 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 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.
The new fall 2010 dashboard might get official release today to all 360 owners.
JTAG users should of course shouldn't update at all (will lock-out old CB and make the JTAG hack impossible), they should wait for a new rebooter.