OpenOffice.org, as of version 2.3, has an option to export documents into MediaWiki format. I've tested it out today on a large book donation, and I'm impressed with the results. The problem was that we had a book donated by the original author in Microsoft Word format. We used OpenOffice.org to handle the conversion between the two formats. Despite my reservations about automatically generated wikitext, the resultant text was not too bulky. Some of the tables with special formatting need to be trimmed, but other then that things went well.
One deficiency that I did find was that I need to upload and insert all the images from the document manually. That is a little annoying, but at least I don't need to format several dozen pages of text by hand. I think that this could become an excellent tool for Wikibooks, Wikisource, and Wikiversity too.