Well, there we go.

If you want to embed using the rich text editor (for example making a thread) then follow the instructions I gave above, if you post from the quick reply box then you don't need to re-edit your post.

Alternately I suspect if using the RTE and you post the embed code into the html itself (using the html button at the end of row 2 in the editor) then you may not need to edit the post afterwords.

Might as well test that last theory (sorry for the vast number of posts)

Ok that works (if you post the code into the html you don't have to edit) Seems to be something to do with the normal editor not adding paragraph tags around the code, wheras using the html function or the quick reply box does.

Alternatively, you can just right-click the video and click on "copy embed html".

Also, if you preview the post I don't think you have to edit.