Just to add to some of the excellent advice being offered here...
As a dev in a small company that does largely gov't contract work in the 6-24 month range, I find that my time is often split between development and writing status reports (right up there with writing documentation, only worse!) Having a wiki to slap down unorganized thoughts and notes as we go along has made report-writing a lot less painful (not pain-LESS, but better all the same).
Further, if you're already in the Mediawiki world, you might want to look at SemanticMediawiki. It allows you to take the organization of your data to another level by semantically tagging it. That doesn't mean a lot on its own, I know, but I can tell you (for example) that it can drastically improve the relevance of the data returned from searches. It is definitely worth a look.