I doubt they will close the entire park on that day given other things planned (like MVMCP). But blocking off Fantasyland for portions of the day is certainly a possibility. Attractions and even parks close as-needed for events and gatherings. Read earlier that Hollywood Studios is closing early today for an event.
In general, I don't recall any great pomp and circumstance accompanying attraction openings. Crowd control is always an issue. And from Disney's perspective, there's a risk that the crowd will not present the image they wish to portray to the media (disinterested, not enough people, etc.)
Is this opening any more anticipated than what I witnessed in 2005? Hard to tell. It wasn't just Soarin but also Lights, Motors, Action and Stitch's Great Escape being heralded. The Epcot event was quite elaborate based upon the preparations we saw earlier in the day. They had professional sandcastle builders, dozens of hang gliders lining the lawn outside of The Land pavilion. There were over 200 media outlets present with many doing remote radio broadcasts from tents lining the park walkways. As the event began we could see a team of skydivers descending into the park from our room at the BoardWalk.
December 6 is for the media and I suspect it will be a highly controlled atmosphere. They may call it a Grand Opening but it's really a Media Opening. Probably no park closure but blocking off Fantasyland is a distinct possibility. And if the public is invited to participate in some manner, I would expect it to be a modest ribbon cutting ceremony with perhaps a couple of speakers.