MediaWiki's syntax is extremely complex and weird; switching dynamically between this and a WYSIWYG mode is not going to be trivial. Furthermore, we have things like dynamically transcluded and parametrized templates which a) cannot be easily rendered as WYSIWYG, b) can break.
Could someone who has GreaseMonkey installed post a quick evaluation of the state of http://demo.wikiwyg.net/wikiwyg/demo/wikipedia/ ? Does it convert accurately back and forth between MediaWiki's syntax and WYSIWYG on complex pages? How does it deal with templates?
Erik