Age Of Conan are going to go about that! iam sure this will also
Although AoC might go about it differently, you can either play on an Xbox360 world only or else choose to play with Xbox 360 and Pc versions.
For the PC user it will be alot easier to navigate around the map and use hotkeys which gives the PC owner at an advantage. So I wont be to surprized if they dont allow Xbox Users play against PC users....it would be good if they gave you the option and let the gamer choose what he/she wants.