WordPress deleting iframes

I want my clients to be able to embed content easily, so I’ve told them how to do it, enter the embed code into the html section, flip back to visual and continue editing, or write whatever, then flip to html and insert the code where they want it.

Problem is the, as ever, troublesome TinyMCE editor deletes iframes almost on a whim. If you put it in html is deleted it when you flip back to html. If you put it in then publish it keeps it, but then edit the post at all it will delete it.

I thought wordpress was doing away with this ridiculous restriction? When does all this having to mess about with TinyMCE to be able to post what you want to end?

How can I stop wordpress doing this?