From the FAQ:
That particular answer seems to of changed recently though; I recall it saying before that the major talks would be recorded and publicly available, but the smaller break out sessions would not be. I would imagine it would be in Valve's best interest to make all the main talks public, as there are plenty of devs out there who can't make it to the conference who could benefit from the information that will be provided there. The more people who know and embrace the info they are trying to get out there the quicker the adoption of Linux/SteamOS and their related technologies will be.Q:
I missed a session, will they be archived?
Yes, sessions will be recorded and available for your reference.