It seems like there's no easy answer to this issue, but I have found that I can use the method I used in PPT 2007 to insert a video. There are a few video's on Youtube that walk you through step by step - the key is in copying the video URL, then modifying
the URL before you add it to the Movie properties. Here's one: http://www.youtube.com/watch?v=BevOmZtKQ_w
Here's the code I copied from Youtube according to the instructions (but look at the similarities of the URL of the video, and what you put into the Movie properties below):
<object width="420" height="315"><param name="movie" value="//www.youtube.com/v/BevOmZtKQ_w?version=3&hl=en_US&rel=0"></param><param name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="//www.youtube.com/v/BevOmZtKQ_w?version=3&hl=en_US&rel=0"
type="application/x-shockwave-flash" width="420" height="315" allowscriptaccess="always" allowfullscreen="true"></embed></object>
Here's what I put into the Movie properties of the Shockwave Flash object: http://www.youtube.com/v/BevOmZtKQ_w
When I put the presentation into Slide Show mode, it works like a charm.