THIS ADDON IS NOW DISCONTINUED DUE TO ANOTHER BETTER ADDON BEING CREATED WITH THE SAME AND MORE FUNCTIONALITY AND A UI
head here to get it:
http://wow.curse.com/downloads/wow-addons/details/lushgearswap.aspx
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This addon simply changes your gear from the set "PVE" to "PVP" whenever you enter a PVP zone or area, and back to the PVE set when you leave a PVP area.
Click here to watch a short movie of this addon working (because i can't really screenshot it :P)

To use this addon, you must make 2 gear sets. one called "PVE" and one called "PVP"

How to use this addon

1. Install the addon
   
2. Create 2 blizzard equipment sets, one named "PVE" for normal gear, and one named "PVP" for your PVP gear
   
3. Give it a try! On entering a PVP zone or area your gear should automaticcally be swapped to your PVP set and the auction house style ding sound should be played to tell you that it has been changed. On leaving a PVP zone, your gear should go back to the PVE set.
   

If you want, open up the EMAC.lua file and edit the first 2 lines which read

local PVP_set = "PVP"
local PVE_set = "PVE"

you can change the 2 names inside the speech marks to whatever you like and those are the sets which EMAC will look for when changing to a PVP zone and back.

FAQ

Why not just use Outfitter?
Even though outfitter is brilliant, outfitter lacks the ability to use the inbuilt equipment manager system.
If you have chosen to not use outfitter, I would recommend installing This addon here, for an outfitter type set menu for the blizzard equipment manager

Why do i care about the inbuilt equipment manager??
Put it this way, a few days ago, the WHOLE of my savedvariables was corrupted. As a paladin i have several sets of gear, and where were my outfitter gear sets? No-where to be seen.
The blizzard equipment sets are saved on the blizzard servers, just like macros, so which ever computer you access your character from, the sets will be there still.

But this lacks so much functionality compared to outfitter!
Working on it. I don't hope to add as much stuff as outfitter, but i do hope to add more to this addon eventually as I learn to do so (this is my first WoW addon S:)

This addon was made using mainly the same code from http://wow.curse.com/downloads/wow-addons/details/equipchanger_zone.aspx created by Beutju. All I did was to change it to a PVP version and change the zones. I may in future add features such as addon menu integration and Raid/dungeon zone changes.

Any helpful feedback is highly welcome. If you find any PVP zones which aren't included, please post them here.