In case this is helpful to anyone, I experimented today with embedding a YouTube video in a Unison course. Here are the steps to take to get it to work:
1. Add a "External SWF default" page type (this allows you to integrate an external Flash movie into the course...either one you've uploaded into the Media Manager or one that is 'somewhere out there' on the web)
2. Copy the "embed" code from YouTube associated with the movie (usually to the right of the video)
4. In the form associated with your new page in Unison, go to the 'SWF' part of the form by clicking the heading 'Step 2: SWF'
5. Double-click directly on the text that says 'Click to the right to select a file' (this will let you fill in the form field manually rather than through the media manager)
6. Paste in the link to the YouTube video
7. Save and preview by clicking 'Preview the Whole Course' in the upper toolbar (the page-level preview doesn't appear to work with YouTube videos so you have to preview the whole course to see it work).
This little section of the embed code is just slightly different from the video URL. But as long as you use just that little section of code, it should work.
The easiest way to pull this little part of the code out is to copy and paste the entire embed code into a text program like Microsoft Word, then copy and paste just the section of code in quotes after the embed src= part (do not copy the quotes).