Today I wrote an extension for emacs that lets you view pdf, ps or dvi documents in emacs. It’s named doc-view.el and I posted it to the emacs-sources mailing list (here it is). But I recommend that you get the latest version by cloning my Git repository.
A description of doc-view.el and how to get it can be found on my homepage.
UPDATE: doc-view is official part of Emacs and the default viewer for DVI and PDF files.