Well, I would be more in favor of having a built-in code highlighter instead of a WYSIWYG editor. I think it'd improve readability, especially with the new endnote syntax threatening to take over some articles. I was going to propose doing something like this for the Summer of Code, but I just couldn't think of an elegant way to implement it. Using TinyMCE for code would require converting HTML-encoded HTML/wikitext into HTML/wikitext, and a feature using Flash or Java probably wouldn't be that popular with users.
We have actually implemented this kind of WYSIWiki (What You See Is Wiki) editor as a Java applet called WikiWizard for the JSPWiki. We will also be presenting it at Wikimania if our proposal is accepted. I have already talked with Brion Vibber about integrating it into Wikipedia and we would be happy to do that, but we are now waiting for the Wikipedia wiki markup spec to be finalized. In the meantime, we are continuing to debug our software and add new features. One of its more interesting features is the ability to copy from Word and paste formatted directly into the applet and have your content automatically pasted in as wiki syntax.
You can see our project details at www.wikiwizard.net. I am currently working on the final steps to make it easy to install and run within JSPWiki as well as the ability to add other editors to JSPWiki. I have almost finished coding a feature to let users upload multiple files simultaneously (for intranet use). Feel free to contact me privately for more details about WikiWizard.
Best wishes, Chuck