Pooya's onto something here. He talks about tagging powerpoint slides to allow for easy find-and-paste into new presentations. He's proposing it would be a feature of Powerpoint.
But why not make it more general and have it be a cross application / medium / location capability? You can tag anything if you have a reference to it. del.icio.us , for example, tags URLs. To make it general, all you would need is a URI for whatever you're trying to tag.
Imagine for a moment we had unique resource identifiers for all of our resources, from mp3s to pictures to individual slides in a powerpoint presentation. Now imagine a little helper app that would let you tag whatever it was you were viewing at the moment. Now put that together with desktop search, ala Google, Yahoo, and Apple. Voila, you can tag and find everything. To finish it up, we'd need the enable applications to understand our URIs and go to them.
Ok, so the URI for anything part is quite a stretch. But keep in mind, we don't have to get it perfect. For things that live on my desktop, they can be unique just to me. For apps that already provide / deal with ids for things they manipulate, we'd simply adopt those.
It's a bit far fetched, but keep in mind Longhorn is planning to RSS everything. That means MS will be thinking of a way to refer to things. So URIs (or something close to it) are on the way…