We wrote a new WordPress plugin and created a page for it:
By default, WordPress’ WYSIWYG editor, TinyMCE, will strip out of your Article and Page HTML code any elements that are not defined as “valid elements”; this can be extremely annoying (especially if you want to include iframes).
This plugin will allow you to extend what TinyMCE defines as “valid elements”. By doing so, TinyMCE will no longer remove, delete, or strip-out the additional elements and attributes that you specify.
Check it out!