Everything at your event communicates a message to your audience. The environment (and I'm talking room setup, not mother earth) is no exception. It can send plenty of messages to your attendees; elegance, professionalism, high production values, same old boring meeting as last time...
That's right. When an audience walks into a room that is staged (albeit with thematic differences) exactly like an event-as-usual, you send the message that it'll be an event as usual. Their brains prepare for the same old death by PowerPoint, the same old line of speakers, the same old cocktail hour in the evening and golf tournament on the second day. If you're making the effort to produce a transformational event--like on the audience has never seen before--better start with the environment of the room.
Now I'm not saying that the room needs to be fancy, overproduced or expensive, it just needs to communicate what you're going to expect out of the audience/participants.
Rounds instead of classroom/theater seating: Send the message that it isn't just a sit-down-and-listen meeting. There will be collaboration, interaction...and intimate experience in an audience of possibly hundreds. Encourage people to get to know their round-mates right away, and even consider putting kinesthetic devices (a.k.a. toys, notebooks, play-doh) on the rounds.
Make use of peripheral visuals: Use the sides of the room to reinforce your message. Put up key points, slogans and sayings, a collective "sharing" board for insights, a road map of the event, etc. Not only will these make an impact upon entry and allow attendees to immediately begin to engage with the event, but throughout the day/s they'll reinforce messaging both consciously and subconsciously.
Pay attention to the music: Most events are preceded by some sort of walk-in music. Usually, it's some sort of popular blend that the A/V crew might have on hand, and it's not something that is typically planned out to a great degree. Leading the event with purposeful, high-energy, positive music that gets people clapping/dancing/chatting as they walk in can set the tone for an interactive event. (Just don't rely on energetic music to carry attendees past the opening if the rest of the event isn't engaging.)
Control the entrance: Don't just have attendees trickle in willy-nilly, chatting or checking their Blackberries as they mull about and wait for everything to get started. Keep the doors closed until people are mostly there. Have leaders in the room already, greeting the attendees and directing them to tables. Have PowerPoint branding/messaging up directing attendees to stay on their feet and clap along to the music.
Of course, these are just *some* suggestions. But whatever you do, if you don't want to communicate that this is an event as usual, don't start your attendees off in the "usual" environment.