Angry Pixel has stated that Halo Wars would be cross-platform, even though there's been no announcement.
"Halo Wars, touted as an Xbox 360 exclusive, is also apparently in development for the PC. But that's not all - apparently the game will also feature cross-platform play, a la UNO and Shadowrun, between gamers on the Microsoft console and their PC counterparts," the site reports, citing one of its "friends over at Microsoft."
When IGN contacted Microsoft, they were told "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," Microsoft said."
Harter Ryan, the executive producer for Halo Wars cut to the chase when he said "There will be no PC port," when interviewed by IGN at the gaming convention in Germany last year.