The fine evince hackers, with special mention to Carlos Garcia Campos, have been of great help during the last week as I was trying to get upstreamed our sugar-evince fork.
Sugar had to modify Evince so we could link python programs against it and build the intended user experience, quite different from that in GNOME. Several people (Marco Pesenti Gritti, Reinier Heeres and Daniel Drake) had worked on it during the last two years, though in the rush to ship a complete learning environment, never got to upstream the changes. Check out the design mockup by Eben Eliason of our document viewer:

So Evince 2.25.5 is going to ship with libevview and I'm going to ask for an exception of the API freeze so we can get the python bindings in the next release of gnome-python-desktop. Then you will be able to write something like this and get your own document viewer:
import evince
import gtk
w = gtk.Window()
w.show()
e = evince.View()
w.add(e)
e.show()
document = evince.factory_get_document('file:///home/tomeu/Desktop/comic.pdf')
e.set_document(document)
gtk.main()
What has this to do with GNOME Mobile?
GNOME Mobile is a platform for other people to build their own software on. Most of the solutions that people base on GNOME Mobile will be able to run unmodified GNOME applications, but given hardware constraints or different intended user experiences that won't happen too often.
Platforms like Maemo have had to maintain their fork of Evince and apply patches to it so they can provide a document viewer that conforms to the Hildon UI guidelines. I can only guess that this is very expensive for Nokia, and what about other vendors interested in basing their software on GNOME Mobile? Will they have the resources needed to maintain forks of all the GNOME applications they wish to reuse?
So I hope other people will feel attracted by the advantages of exposing widgets out of apps and we can work together so as time passes more and more GNOME apps can be embedded and new UIs get built around them.
And GNOME 4.0?
I have no idea what GNOME 4.0 will be, but I know that lots of people (and organizations) in the GNOME world are not resigning themselves to build a Windows-clone and are willing to explore new user experiences that help computing to bring more benefits to more people.
By splitting base functionality to shared libraries and making easier to build new UIs around them, we have a better base on which to play and experiment, until the day will come ;)