Google says they're...

<<.. extending the YouTube Partnership Program to include individual popular videos on our site. Now, when you upload a video to YouTube that accumulates lots of views, we may invite you to monetize that video and start earning revenue from it. To determine whether a particular video is eligible for monetization, we look at factors like the number of views, the video's virality and compliance with the YouTube Terms of Service. If your video is eligible for monetization, you will receive an email and see an "Enable Revenue Sharing" message next to your video on the watch page, as well as in other places in your account.>>
http://googleblog.blogspot.com/2009/08/in-future-everyone-will-monetize-their.html

[moved from a blog post just now as I noticed it was older after posting... still getting used to not having access to Blogspot (I didn't immediately see the post date this way).]