This exploit mainly applies to whichever side is attacking, although I guess it can be used if you are defending (just a little less practical). There are several mods that aid in doing this by knowing when WG is about to start. You can find any of these mods from searching your favorite addon site for Wintergrasp.
Anyways, the exploit begins about 5 minutes or so before WG starts. No, this is not a tap the orb from the orb room repost. Instead, screw the orb, but how about helping your team get into the fortress easier.
What Blizzard failed to do, was kick people off the walls of the fortress when WG begins. If you are a defender and are sitting in one of the towers waiting for a turret to spawn, you will be ported out. But, if you are on the attacking side, and sitting on one of the walls waiting for a turret to spawn, you will stay right where you are when WG starts. You can easily fly in from Icecrown and just time getting kicked off your mount with landing on top of an outer wall.
Of course, this works much better for ranged classes, but melee classes (and stealth classes) can seriously do some damage too. Once WG starts, start attacking the turrets on one side of the outer walls (east side or west doesnt matter, they both work). You can easily take out 2 turrets before the game is even a few minutes in. From there you can jump into the fortresss and head to the towers or whatever your heart pleases.
I know this is not a major omg I.W.I.N. exploit, but ohh well. Sorry for those that read this far and I did not give them a way to one-shot every class in the game through a secret code.
I would also like to point out that I did search and the search cooldown really sucks for leechers like myself. Ohh well, I will deal with it.
If you would like SS's, I can provide them later, but I have to get going to work for now.
Edit: After some more testing, you will be ported out if you are on top of one of the towers. Just like the defense, if you wait inside a tower for a turret to spawn, before the game starts, you will find yourself outside of the fortress. As long as you stick to staying on top of the large square cornerstones of the wall, you will be fine. When the game starts, you will still be there ready to snipe a few cannons to allow your team some easier access.