No, it's not some preorder bonus. But quantities are limited. Sega's offering 40,000 codes to play the Bayonetta demo early on Xbox 360, but you have to be a registered Sega Pass member to be eligible.
Here's the deal - once you register (free) for a Sega Pass, go back to this site on Nov. 9 and register to get your code. Registration for early access will be open until Nov. 13, but given the first-come, first-serve nature of this offer, my guess is it won't last that long.
Again, this offer covers the Xbox 360 version of the Bayonetta demo.
Bayonetta Demo Offer [Official Site, via Ripten]