you’ll probably want to use some javascript to do a bit of plugin detection and then inject the video into the document if the plugin is available, or write a quick “you need to get…” link so people can grab the plugin.
if your video’s are flash, as youtube vid’s are, you can look into swfobject.js. you can run through flash/version detection and then either add your video or replace with an download flash player statement.
if you’re using the mootools framework (1.2.3) i have some plugin detection stuff that could help, but if you are using vanilla JS, just grab swfobject, it works.