It is not entirely known on whether this will be an event or an actual raid. But we do know that the seige and taking down Garrosh will be the final stage of Mists of Pandaria. If this is true I seriously, seriously hope that Blizzard plans to update/change Orgrimmar significantly for the raid. Look how ill-received Dragon Soul was. It was just a recycled area, with mostly recycled mobs that we've seen before. I love LOVE Mists of Pandaria so far. Much more than Cataclysm at least. But I think it will be a big mistake on Blizzard if their last raid of Mists of Pandaria is running around in same old Orgrimmar. My idea on how they could change Orgrimmar up a bit is how Garrosh updated Orgrimmar for Cataclysm he could do even more in the final patch. The front of Orgrimmar is still under construction so that might be a clue that the upgrade is still not complete. Imagine a giant metallic badass Orgrimmar?
It is too early to really tell whether this will turn out this way. But will you care if its another Dragon Soul fiasco? Do you think it will be? Or do you think it won't even be a raid but an event (scenario, world event, etc.)
EDIT: Another idea came to mind. With the offical blizz quote talking about the Siege: "Garrosh has become corrupted and has been doing some really nasty things beneath Orgrimmar." Maybe its possible the raid will take place beneath Orgrimmar? They did seem to get rid of the bug where you could go underneath Orgrimmar in Cata.
It's been said that the Siege of Orgrimmar is the final raid of MoP, which will involve unseating Garrosh.
It's also a popular theory that the end of the raid will involve going into the caverns underneath Orgrimmar, some part of Ragefire that we haven't seen yet. It'd be an ideal spot for the final showdown - after all the spectacular new dungeons in MoP, it'd be a shame if the final act simply involved a phased version of Orgrimmar, even if the construction going on is finished up and everything gets outfitted with black iron.