Cataclysm is officially live on North American servers! I was able to log in after about 8 minutes of connection issues. Hopefully this means that NA players will have better luck getting into the game than the European players did during their launch about nine hours ago.
If you're looking for details on the expansion, Wowhead has all sorts of information to help you out. You can also read Blizzard's concise list of Cataclysm features from their launch Q&A after the jump.
So what's the first thing you're going to do? Create a goblin or worgen? Start leveling to 85? Do something else entirely? Personally, I'm going to take a moment and enjoy flying around Azeroth. See you in-game!