It's rumoured that Windows Vista will include a gaming service called Panorama:
Apparently, Panorama will be a part of Windows Vista and will act essentially as Xbox Live Arcade for Windows. PC gamers will be able to not only play Live Arcade games, but will also be able to engage in multiplayer gaming versus gamers playing on the Xbox 360 version of the service. It does not appear that Panorama emulates the Xbox 360 environment, which suggests that developers creating cross-platform compatible Live Arcade games may have to release two versions, one for each platform. However, Microsoft tools such as XNA should expedite this process.