Matthew,
as you know, one can include relative easily forum contents via adp includes into xowiki pages. The ajax and fade-in / fade-out effects will certainly work with this as well, since this works with all kind of xowiki pages.
however, this does not make a xowiki a good forums application, since all links of the forums will point to the forum apps, etc.
i would estimate that it is more work getting the forums into xowiki than writing an xowiki forums application. One could use e.g. the page order for representing the threads and replies, and make a includelet similar to the "table of contents" doc in http://www.openacs.org/test-doc/
-gustaf neumann
