It looks like you will be able to stick the Xbox Magazine demo disks in your Xbox 360, but don’t expect to be able to play any original Xbox content on there, as if you recall, the Xbox 360 uses an emulator to run older titles, and no one in their right mind would get that emulator to work for every demo out there.
“…starting with the next issue (January 2006), [the discs] will have Xbox 360 demos that can play on Xbox 360 and Xbox demos that will play on Xbox 1. All of this will be on one disc. However, our Xbox 1 demos will NOT work on Xbox 360. You will need an Xbox 1 to play them. “
source: Xbox Solution