I have been looking at porting a old content editor I wrote from WinForm to SilverLight and hit the problem there was no HTML editor control available in the standard Silverlight 2 control set. Roll in the excellent SLExtensions controls on CodePlex, to save the day.....
Now I did hit one problem with the HTMLEditor, that was addressed very quickly in for support forums. The point to watch out for is that for the HTMLEditor control to work the Silverlight object must be loaded into the web page with the setting to be windowless
<object id="appId" data="data:application/x-silverlight," type="application/x-silverlight-2" width="100%" height="100%" >
<!-- all the other parameters -->
<param value="true" name="windowless"/>
If you just add the object with default settings to a new web page, or let Visual Studio generate a dynamic test page then this is not set. You end up rendering the editor but cannot enter text.
Hope this saves you some time.
Read the complete post at http://blogs.blackmarble.co.uk/blogs/rfennell/archive/2009/02/25/slextensions-htmleditor.aspx