I'm wrapping up listening to the most recent episode of DradCast, hosted by WordPress co-founder Matt Mullenweg. Matt speaks of what WordPress can be ten years from now, and included in his response mentions, “when you think about it, we're kind of building a web operating system.”
The phrase strikes a cord with me, and I think really encompasses what the future of WordPress can really be for the web. It's a tool, and a framework for developers to do as much as they want with it. And WordPress itself doesn't have to be the focus of the app. In fact, at its best, Matt describes that WordPress would be relatively invisible. The whole interview is quite good.