From the YouTube blog: As we learned in kindergarten, sharing is good. We want you to be able to do more of it with your videos. With a simple one-time log-in on the upload page, you can now have your YouTube account automatically update your Facebook friends, Twitter followers, and Google Reader shared items when you upload a video.

If you change your mind, you can shut it off at any time. And of course, for those videos that you'd rather not share, mark them as private and they won't be sent to any other site.