As they didsn't seem to exist, I have created two Mail templates to make it easy to share YouTube and Dailymotion videos within Mail.app. You can find everything you need on this page within my site.
Double-click on Dailymotion.mailstationery and YouTube.mailstationery to install them. Then place the Create New Dailymotion Message.scpt and Create New YouTube Message.scpt scripts in your user's Library » Scripts folder to get to them on the menu bar. Launch the relevant script for the site you're viewing, then copy/paste the permalink for Dailymotion or the URL for YouTube, and your email will be ready to send.
Feel free to improve these to suit to your needs.
[robg adds: The linked page is in French, but it's fairly easy to see the four items that need to be downloaded. In case the original site ever vanishes, I have mirrored all four files here on macosxhints.com.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090726053208519