I always hated how owning a different electronic device meant you couldn't interact with others on a different one. For example, my cousin owns a PS3, while I own a 360. Yet, we can't play the same game online with each other simply because Microsoft and Sony don't want that capability (granted there are also technological differences in how the two do multiplayer as well) But he and I would, and we would pay extra for that feature.

But I imagine it's possible across portable devices that allow the installation of an application with a multiplayer component. It just takes talented developer(s), a great vision, and perhaps a budget for the setup of network hardware and network related knowhow if it's done online (as you pointed out due to the iPhone's limited bluetooth support).