I haven't seen a true WYSIWYG yet but there are a few places you can start: phpslash.com & phpnuke.com. Not sure what you are specifically looking for but both are open-source Content Management Systems and are at least worth checking out for feature and ideas of what you want.
I've read a bunch of articles in the past of how to do your own but they've always fallen far short of what we would consider WYSIWYG. I guess unless you have a Java Applet or an Active-X PlugIn, this is still a cryptic solution to develop yourself. If you find 'GOOD' articles on how to do your own, please post them back here.