This "breaks the page" – see Vimeo embed codes.
Did this ever get solved? Just ran into it.
Try putting a space between <iframe ...> and </iframe>.
I've been running into this as well. A space doesn't solve the issue when using kramdown.
Switching to rdiscount seems to solve my problem. I've filed a bug on kramdown: http://rubyforge.org/tracker/index.php?func=detail&aid=29490&group_id=7403&atid=28673
kramdown only recognizes correct XHTML tags, not HTML5 tag like <hr> or attributes without values (although it tries to fix such things if possible). Therefore the iframe tag in the rubyforge bug does not work in kramdown.
As far as kramdown is concerned: The fix for this issue was introduced in kramdown 0.11.0, released October 2010
Add workaround for bug causing footer to disappear.
This is a problem with your markdown converter. RedCarpet 2 should work.