In case you're still not sure why this is a great idea, consider this: At the moment, your Writely files are stored on Writely's servers, your Num Sum files are stored on Num Sum's servers and your Basecamp files are stored on Basecamp's servers. (If you're not using any of these applications, please read this posting again in two years. Chances are you'll be using one of these services, or a similar one, by then. BTW, why not send yourself a reminder in two years to see if my fortune-telling is right?)
Openomy is an online file system. You can store files on Openomy and access them from any computer. Openomy organizes files and users via tags (as opposed to folders). You can choose to keep your files guarded by Openomy, or allow certain outside applications (of your choice) to do new and interesting things with your data.
So, do you have a separate hard drive for Word, Excel and Powerpoint? No? Then I'm not sure if it makes sense to have a separate file system and storage server for each Web-based application.
I think Openomy is a really, really smart idea. I also think a viable business can be built around it. To enter the market I think they should build one really cool application that is closely tied with Openomy and give it away for free or for as little money as possible. The most obvious application, of course, is to address the terribly underserved market for online storage and remote backup. And while not offering a perfect solution for that need yet, they're already beginning to do just that.