As you might guess from our list of new abilities in Wrath of the Lich King, leveling as Shadow in Northrend is basically the same as leveling as Shadow in Outland. The only difference is that in Northrend, you get to be a little more reckless thanks to Dispersion. Being reckless is fun.
If you've leveled Shadow before, you probably have used the "pull way too many monsters, tab DoTs, fear, and run around screaming like a fool while they slowly die" tactic. It's tried and true, so I don't know why you wouldn't. You can level pulling only one mob at a time, but why would you? Flailing your arms around in horror is much more efficient, and Dispersion makes it even better. Shock and awe, I know! Dispersion is good!
Let's take a look at the talent spec I've been using in Wrath, shall we? Note that when you look at this, it's a talent spec I am using because it works for me. Leveling is one of those times where you can get away with your talent spec being wholly different. Pick what works for you. This spec works for me.