I searched the forum for any thread like this, but i found nothing, so i guess this hasn't been posted before.
Here's the video. Not high quality, but you should know how to get into Dalaran after watching this:
http://www.youtube.com/watch?v=gJGqBf9RX9Q
Okay, i'll try to describe it as clearly as possible. It's pretty simple: Mages can blink into Dalaran (The big purple orb thingy in Alterac). You can't blink trough the orb everywhere, but there is a place. On the North-Western side of the orb there are ruïns of a tower (a tower like the one in westfall). Enter the tower, and get as close to the purple wall as you can (On the left side of the tower, but still in the tower). Then, simply blink and walk forward, and you're in! This is not really an exploit since there is nothing in dalaran, but it's fun to do it and amaze your party members.