The RIA world is still kind of a wild west of ideas, and as a result of that people try to define what "Rich Internet Applications" are. I think that's great, and I don't expect to be the one who creates the definition, but I wanted to throw my two cents about RIAs and cross platform.
I think cross platform RIAs are important, I think they're in the spirit of the web and I think reaching the widest possible user base that you can is important. But I don't think that an RIA needs to be cross platform to actually be classified as a Rich Internet Application. I think there is far too much innovation in the space to limit the definition like that. In addition, how do you define "cross platform"? Is it running on Windows and a Mac? Running on Windows, Mac and Linux? What about all the different flavors of mobile devices?
I think it's a good discussion to have, but when I take a look at applications like the New York Times reader, I don't see how you can try and not call that a Rich Internet Application. It's abstracting web content, wrapping a very interactive, branded experience around it, and allows you to take the information offline with you. Sure, it's a windows application, but it's the showcase app for Windows Presentation Foundation and a high quality RIA.
In the end, I think it's about creating those great experiences and breaking out of the browser that defines a Rich Internet Application. We've viewed the web through the pinhole of browser for way too long. With technologies like XML and micro formats that bring data to us in formats we can use, we're able to build powerful RIAs with a lot of great content. That's the goal, and I don't think it matters if that experience is available only for one platform. In the end, economics wins out and if customers demand cross platform, companies will follow.