First contribution attempt, I would tell you not to flame but I know it's coming
Someone posted some time ago how they would create client lag on their PC right before a loading screen (on the boat) to the point where they would become desynced with the server. Upon relogging they were able to run around "discovering" new land an such. Well, I stumbled upon this method and it works fairly well to achieve. Lets me walk just about anywhere without a loading screen (because I can't reach the portal to get in), "discover" new areas, etc. Perhaps it will be useful to you.
I use a warrior to do this, but perhaps other classes might be able to as well.
This is what I do:
1. Anywhere I would like to get underground I find a mob on the bottom of a slope, like a hill or a shoreline.
2. I set myself at the top of the slope, and use "charge" down at the mob.
3. As soon as my character's arm reaches back to strike the mob, I physically disconnect my lan cable. See Update Below
4. Once I get the disconnected message, I reconnect my cable and log back in.
5. If you timed it right, you'll notice your character name in the selection screen does not display the last last location you were at (ex. Ashenvale) under it.
6. Log back in and and notice that all mobs, copper veins, etc are above your head.
7. Now you can explore somewhat without the worry of mob attack. Although if you position yourself correctly... sometimes you can attack monsters (and get away quite easily) Unfortunately if you or they run off, they evade.
Now to you, you appear to be on land and all mobs are hovering. But to other players it looks like your walking underground. On my GFs computer (who was watching me) it appears that I was jumping in and out of the floor of the beach we were on.
Also, even though I was clearly underwater server side (and to other players), I would never run out of air as long as I was above water on my client's side.
Occasionally I've been able to resync myself. I did this in one area where there was a horde guard post. I ran under the mobs and jumped in a lake behind them, relogged, and now I was in the water back to normal.
So I haven't found anything particularly cool while doing this, but hopefully you guys will able to get somewhere
Here's a video from my viewpoint of what it looks like.
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Tested on a HP Laptop 1.8GHz 2gigs RAM on a slow 768Kbps cable connection.
Update: This might help with timing it right: You want to disconnect your connection right before gaining aggro. If you do it perfectly, the monster might even turn to face you! I realise this might be difficult for people with desktops, but if your crafty (or have a laptop), you might be able to figure out an easier method.