Sorry, I don't believe there are any web WYSIWYG ReST editors :(
You might be able to use something like TinyMCE (preferably something that outputs very clean HTML), then convert the resulting HTML to ReST using html2rst.py.
Another cheap solution would be to re-use something like the Mediawiki editor, customizing the buttons it provides like this:
<script type='text/javascript'>
/*<![CDATA[*/
addButton("images/button_italic.png","Italic text","*","*","Italic text","mw-editbutton-italic");
addButton("images/button_bold.png","Bold text","**","**","Bold text","mw-editbutton-bold");
addButton("images/button_headline.png","Headline","","\n=============","Headline text","mw-editbutton-headline");
...
</script>
This is not WYSIWYG though, it's more like the editor here on SO. You'd be editing the document as raw ReST.
Do you want the live preview like StackOverflow has? If so, you'll have to either render the ReST in Javascript, or send it to the server and update via Ajax, and I don't believe there's a JS implementation of a ReST renderer.