Updated: The new total opt-in for Facebook Beacon that Mark Zuckerberg alluded in his public note today is not now totally preemptive. When Zuckerberg's note was sent out, if you hadn't been to a Beacon partner site, you got this message in the Facebook external Web site privacy control:
Privacy Settings for External Websites
Show your friends what you like and what you're up to outside of Facebook. When you take actions on the sites listed below, you can choose to have those actions sent to your profile. Please note that these settings only affect notifications on Facebook. You will still be notified on affiliate websites when they send stories to Facebook. You will be able to decline individual stories at that time. No sites have tried sending stories to your profile. When they do, those sites will appear in a list on this page.The "bug" has been fixed, so Facebook members can preemptively opt-out of Beacon.