Just seen this on xboxscene, now m$ connfirm it themselves, the c$$** LOL

New Xbox
>> It has already been in this news several days ago , but now it's also confirmed by MicroSoft themself.
The new Xbox (let's call it v2) is coming.

From gamespot :
The newly configured units were first spotted in Australia, where Xbox hackers spotted slight changes to the main circuit board that likely will make it impossible to use current "mod chips"--add-ons that bypass security measures built into the hardware.

The new configuration reportedly also uses a different type of BIOS, the basic software that controls the console's operation, and eliminates the fan that had been used to cool the console's custom Nvidia graphics processor.

An Australian hobbyist posting on the Xbox Hacker site said he tried all currently available mod chips on the new Xbox configuration and none worked.
From theregister :
Although Microsoft habitually experiments on the Australians, we do not think this is an experiment. Microsoft sees keeping Xbox closed as a security/revenue issue, and will therefore keep trying to do so. You'll recall it recently advertised for someone in this patch and it's to expected that the company will react to successive mods with successive mod-blocks.

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 DVD Rom 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.
The xboxhacker.net article they are talking about can be found here.

New xbox are already in australia and coming very soon (now) to Europe & USA.
If you wanna make sure you buy a v1 , there's a hole at the bottom of an 'xbox box' , there you should be able to see the manufacturing date. If it's August 2002 or later , buy at own risk. If it's prior August 2002 , it should be a v1. (the date is written in the year-month-day format)

It is unkown yet if they just changed the RC4 , changed the LPC , added more security checks or anything else.
More news 'bout this v2 xbox soon ...