Not entirely unexpectedly, many servers are having trouble staying up and active this evening. In most cases the server itself isn't going down, but rather individual continents. Northrend and the instance servers in particular are taking a beating, and at least on my server, Eastern Kingdoms is dropping like a rock every now and then, too.
If you had big plans for this evening, you may want to reconsider. At the very least, raiders may want to wait until prime time has passed to give things a whirl. Generally, these things blow over as people start to log off for the evening, and there's less of a load on the server.
Blizzard is aware of the issue, so hopefully we'll see a fix for it all soon. I personally suspect it's unlikely things will magically get better tonight, but maybe before the weekend. That would be nice.