Speakers are informed they are going to be recorded. If they have sensitive information, they can choose a track and ask it not be recorded. NANOG has done this in the past, but you should talk to the Program Committee if you are interested in this.
We've had this within UKNOF ... sometimes people do not wish to be recorded, mainly due to confidentiality reasons (ie: advance heads up, or personal thoughts delivered to a specific audience). Occasionally we have been asked to remove recordings at a later date due to changing circumstances etc. We explicitly mention the webcast/records on abstract submissions from memory, and also recently introduced shepherding to help presentations be more relevant (both to the speakers to help them in pushing a $clue or message, to our audience to ensure relevance and to us in terms of protection from litigation, etc). This applies to both submitted AND sponsor talks (the latter being incredibly useful and has shown a major increase in sponsor talk relevance and feedback ratings). People will always mention a lack of recording/webcast for this type of content ... but then arguably that is a driver to attend in person. Thanks Chris (UKNOF PC Chair)