Enable oEmbed support for comments. Requires WP 2.9+.
It's the same oEmbed functionality you know and love from your WP 2.9 post editor, now enabled for comments!
Your comments can now embed content from the following sites:
How do you use the plugin? Simple! Input any URL from one of the listed sites above into a comment.
When the comment is posted and approved, the URL automagically transforms into the embedded content.