You can now add Youtube videos to your profile. It will automatically play the video once you visit the profile. To add a video to your profile, find one at Youtube.com and copy the link of that page.

For example:
http://www.youtube.com/watch?v=bJpQXaFLimc

Now once you got the link, go to your User CP and click on Edit Your Details. Now at the bottom, you'll see 2 text boxes about Youtube. Hit the link in the first box and give it a title at the second. After that, hit Save and you're done.

For a example how it will look like, check my profile.

Have fun

Edit: Auto-play switched off