I hereby declare the article validation function to be operational with basic functions, which means it works but is still full of little icky bugs :-)
To try, use CVS HEAD and set "$wgUseValidation = true ;" in your LocalSettings. What to vote for is determined in Language.php by "$wgValidationTypesEn" (demo is provided in CVS).
On the default skin (no other skins yet, sorry) you'll get a "validate" tab on all articles (no other namespaces), for both old and current versions. On a sidenote, I discriminate versions by their timestamp, as there is currently no other versioning system that works for both cur and old tables, AFAIK.
From the validation page, you also get a link to a Special Page (tm) showing the statistics for the different versions of this article. Breaking down statistics further was not implemented due to privacy reasons and my personal lazyness ;-)
There are plans for some kind of "German 1.0" in the near future, not to mention the English one and the one-volume printed version, so it would be nice if we could get this running soon.
Magnus