Gmail is currently testing a new user interface for previewing YouTube videos in a lightbox interface, but it only works for YouTube's email digests. "Now, when you click one of the thumbnails attached in the newsletter, email messages etc. Gmail now shows the YouTube player in a "lightbox"."
For some reason, this only works when the "Apps Search" lab feature is enabled.
Here is the video demonstrating the feature:
[Thanks, Emanuele, Alex]