Live regions containing links, images, document structures, and form controls

The ARIA spec (and User Agent Impl Guide) is a little unclear on what a
user agent is supposed to do when a change to a live region involves
something other than just text.

For example, what if an alert has a button in it?

  <div role="alert">
    Message sent!  <button>Undo</button>
  </div>

After announcing "Message sent!", should a screen reader say "Undo", "Undo
button", or nothing?

Same question for links, other form controls, and static things like
headings and list items - should the role be announced or not?

I'm seeing quite different behavior depending on the browser and screen
reader combination, it'd be great if we could all agree on what we'd like
to happen.

- Dominic