I heard Southshore will be a horde town in the expansion.
I also heard that there will be a quest chain(s) and going further in the chain will make the world look different. the example they used was the sea rising more and more. I am not sure if the whole actual cataclysm will be a quest chain, but that would be cool.
The most common problem in this is having zones phased for characters on certain parts of the chain, but Blizzard also said that they are looking at a way to make it to where you can invite other people into the phased area you are in... so maybe that will have it work...
Then you run into the problem of an area like Azshara... with it now being a ~L.50-ish zone, but for the expansion it becoming a (phased?) low level area. Since chat channels are not phased you could be L.50 in old Azshara needing help and everyone else in the zone is phased to the low level Azshara.
Speculation without any real facts about HOW the actual cataclysm works out is fun. Will it be a sudden occurence and everyone being affected at the same time and it will hit on the day the expansion hits? Will it be server related... like the opening of AQ and IoQD? Will it be a quest line that only affects the person doing it up to a point? Seeing as Blizzard is doing something like cross-server LFG I don't see why they couldn't do something like old Azeroth being on a subserver while the new Azeroth after (or during) Cataclysm could be on the main part of the server.
I guess since the Worgens couldn't really be let out til the catalysm breaks open the Greymane Wall it would seem that starting a Worgen and doing a chain would open up the rest of the world to you... kinda like how the DK starting zone is... I don't know the specifics of how the cataclysm affects the Goblins, but I am sure it is pretty much the same type of chain that lets them "get out".
I don't remember if Blizzard actually announced HOW it will take place except how the sea rises during a chain. I have only watched a little of Blizzcon and I really haven't been looking for "official" news.
Be kind to me. As I stated... I have only speculated some of this so it carries no actual weight.