Yes, I'd love a way to create my own "meta objects", basically a "named group" like someone else mentioned. It would be a collection of items that I group into one, and if I changed the "root" item all uses of it would change correspondingly. Kind of like Sketchpad back in the day. http://www.youtube.com/watch?v=mOZqRJzE8xg

It's already planned but I'm still adding 3 more votes onto the heap. My 2 cents on this feature:

1. Designing a UI and explaining the behavior are 2 pretty different tasks in my mind, and in terms of the conversations I have with other people. I find it smoother to draw out the UI, and then add explanatory notes.

2. Having the annotations "attached" to an arbitrary graphical element would be splendid. They'd remain "semantically connected" and then could be automatically drawn in a separate layer / mode as "callout boxes" with an arrow pointing back to the visual element.

3. Basically this is an "uber-comment" kind of feature; letting people comment in a more directed way on specific parts of the interface. It would be there to facilitate conversation, and provide longer textual explanations when that works better than showing directly.