Seems there was a bit of a bug with the last WordPress MU (WPMU) upgrade which means if you want to be able to embed Youtube or Vimeo video in a post, you’ll need to update your kses.php file. It was causing real problems not being able to display videos on sites, so I had found the Smart Youtube plugin worked to get around the bug, but obviously only for YouTube, not other video hosting providers.

Anyway, the solution is to add a few lines to the kses.php file, which is neatly outlined on the ClickOnF5 site. Hope that helps other people too!