Unity is supposed to be able to do a desktop spread (similar to pressing Super+W, where you get a view of all windows in the current workspace) and a workspace spread (similar to Super+E, where you see all the workspaces).
Right now it's easy to perform a workspace spread because it's mapped to the workspace switcher, but desktop spread is supposed to become very relevant in Unity at some point, so how will the user access it? Will it be mapped to the workspace switcher instead of the workspace switcher? Or will Unity use an all-new effect halfway the two that exist now?
You will be able to map the workspaces launcher in Unity to any of the Compiz spread / expose plugin behaviours you like.
We are prototyping a new approach which makes desktop spread ("show all windows on the desktop") the default behaviour of that icon in the launcher.
All of the following is subject to change
The two features won't be merged.
[retracted*]. Exposing just the windows of any specific application will he done by right clicking the application's icon in the Dock. (ref)
As for the workspace overview, there will be an icon in the dock showing the four workspaces. Clicking it will result in the effect you described, same as the current Super+E)
By default, Unity will have four workspaces (ref), they will exist alongside the two "Exposé" features.
This screen shot shows both features. The workspace switcher is the last icon in the Dock.
Unfortunately, Unity doesn't use Lauchpad (publicly) to track their blueprints. But the bug reports sometimes give some insight into what's happening.
Also keep in mind that, while the specification is (apparently) done, there are quite a few thing that are just not decided yet, and there may well be major changes until April 2011.