One of the most impressive sessions Jim Rossignol from Rock Paper Shotgun was able to attend at GDC was the discussion of Open Worlds in the context of the development of Prototype, as hosted by Tim Bennison and Eric Holmes of Radical Entertainment. Much of the presentation was focused around the nature of developing open world environments and the way in which systems interconnect to provide options for players within those kinds of game worlds. Interesting stuff, if spider-diagrams of game systems and possible player actions are the kind of thing that draw you in.
The showpiece of the session, however, and the reason many of the audience were there, was to watch Holmes play through a large slice of the game. And, well, Jim think his interest in this game has been validated.