YouTube has introduced a new way to embed videos. The new style uses an <iframe>.
Viewers of videos which use the following embed code style will be able to view the embedded video in one of YouTube's Flash or HTML5 players, depending on viewing environment and preferences:
<iframe class="youtube-player" type="text/html" width="640" height="385" src="http://www.youtube.com/embed/VIDEO_ID" frameborder="0">
YouTube lists environments that support HTML5 here. They just use Flash when it's not supported.
"An additional 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," says YouTube software engineer Toliver Jue on YouTube's API Blog.
YouTube evidently intends to make this the standard embed code for users eventually, but for now they're just previewing it.