Microsoft has made some modifications to the internal design of Xbox in the name of security, the most immediate upshot being apparently that existing mod chips won"t work with the new design. According to a posting last week on the Xboxhacker BBS.
According to the Xboxhacker posting, there are various modifications which may be cost and reliability related (e.g. the video chip loses the fan, and the DVDROM has been switched to a Philips.) The key change that at least impedes the modders is however that a small chip that used to be behind the onboard bios has been removed.
It would seem likely that the BIOS itself has undergone some modification along with this. We"ve subsequently been informed that the RC4 keys have been changed, but that it should still be possible to replace the bios via the LPC bus. Which suggests the latest moves are a relatively minor impediment to the current modders.
No word one yet from the Xbox Linux project on the implications of these changes.