Update: It's true. See here.
According to this PSU piece, the latest Famitsu brings word that a co-op-enabled Resident Evil 5 demo will release on December 5.
The story's tagged as "rumour," so we're assuming this has come from a third party, although there's no link.
There's no word on whether or not this is a Japan-only affair, assuming it's true.
The game's out on March 13 for both PS3 and Xbox 360.