The official Shadowrun forums declare that a demo will be released, er, half an hour ago on Xbox Live Marketplace. So, 2am PDT.
There's no sign of it yet according to Internet hero Kristan Reed, and apparently there's no immediate plan for a Windows Vista version ("We haven't announced [it]").
But we do know what's going in the Xbox 360 one: the first three training chapters, bot and online multiplay, the Power Station level, Elf and Human races, four types of magic, three types of tech, and six weapons including the katana.
Specific details are all on the forum.
Shadowrun - released last Friday - is an online multiplayer game derived from the pen-and-paper RPG series of the same name. One of its headline features is the facility for PC and Xbox 360 owners to play against one another on the same servers.