I hope for google video and youtube results, google might be using some internal techniques for the thumbnails, and that is also quite possible.

But metacafe.com was a third part website, and has a thumbnail view, so after observing the html code of that website I got to know that, its using the above tag to get appear the thumbnail image of the relevant video.

Neways but i am not much sure, if this is going to work out, still I have done an experiment with one of my page using this tag, hope it takes some time to be figured out at google for the relevant keywords, lets see if works out.

thanks
Bulu Pattanaik