YouTube recently announced a new way to place embed code which uses the <iframe> tag.
According to YouTube's API Blog, this will allow viewers to see embedded content through both Flash and HTML5 devices.
The blog also says that this will allow for an added benefit, "of the new embed style is that it will eventually allow embeds to work on mobile devices, which typically use a built-in player instead of Flash or HTML5."
The following video is embedded with the new tag:
Here's an example of what it looks like in code:
<iframe class="youtube-player" type="text/html" width="640" height="385" src="http://www.youtube.com/embed/VIDEO_ID" frameborder="0">
We care for as long as it works on iDevices
via YouTube API Blog