Cross-platform capabilities have been accessible through UNO and Shadowrun, and it's no real shock to postulate that an RTS like Halo Wars could be in development for PC. Apparently Sega's Universe at War will have cross-platform capabilities when it launches on Xbox 360 this spring, so it would seem somewhat plausible for a first-party title like Halo Wars to feature the capability as well.
Update: Predictably, Microsoft has squashed the rumor. "Halo Wars is being developed from the bottom up for the Xbox 360 and its control scheme to ensure that we deliver a groundbreaking console RTS experience. Given that, we have no plans to develop Halo Wars for Windows Vista at this time," a Microsoft representative told IGN.
Read: Fog of War: Halo Wars for PC?
Read: Is Halo Wars going cross-platform?