The shattering has happened. Servers are still down until 11:00 am PST (probably a little later, they're never up when they say they will be.) All the changes to the old world are here. You still can't fly in Azeroth until you actually own Cataclysm and buy the skill to fly there though. Also, portals in Dalaran and Shattrath have been replaced with class trainers. I guess Blizzard doesn't want people still hanging out in Dalaran because of the convenience of the portals. The Azeroth flying will go a long way toward making travel tolerable again though.
If you try to log in right now, there's a small patch, and then you get the Cataclysm login screen.
Also, new class and race combinations are available (obv not including worgen and goblin), new gnome/troll starting areas, and basically a ton of other stuff. Here's a list from mmo-champion.com of all the stuff that is and is not in today's patch:
What is in:
Portals in Dalaran/Shattrath Removed
Class trainers in Dalaran where portals once were
New race-class combinations (excluding worgen/goblin)
New Gnome/Troll starting areas
Changes to existing zones
New cata load screens
New cata intro cinematic
New cata login screen
New music
City Quartermasters, with rep tabards
Bug fixes
Class balancing
Druids, paladins, priests, and shaman will have their talent trees reset
Experience required to gain levels 71 through 80 is being reduced by 20%
New tamable hunter pets (Monkey, fox, dog, and beetle, as well as new skins for existing pet classes.)
What is NOT in::
Creating worgen/goblin characters
Worgen/Goblin start zones
Archeology
Professions past 450 (Illustrious Grand Master)
Guild leveling
Guild achievements
Eastern Kingdoms/Kalimdor flight
New Zones (80+)
I'm pretty sure the revamped Azeroth dungeons are all in too. That means you can go check out the new Deadmines and Shadowfang Keep, but only the low level versions will be available.
The stuff that is not in yet will become available on midnight PST, Tuesday Nov 7th ("monday night", but technically it will be Tuesday), but only if you have a Cataclysm flagged account. For anyone who wasn't aware, you can buy a Cataclysm upgrade through the Blizzard store and preload the Cataclysm data now, also bypassing the need to run and get a copy, or upgrade your account through the account management site.