Valve promises a "relaxed, off the record environment" where developers can attend lectures from industry veterans and participate in round-table discussions. "Off the record" means there will be no press at the event, but hey, there's free wi-fi. The Steam Team will be on-hand, along with SteamOS, prototype Steam Machines and the Steam Controller.
Valve is emailing registration codes to developers, and there is an attendance fee of $95 per person. The specific schedule, speakers and sessions will pop up here closer to Steam Dev Days.