A very simple addon that allows you to bind a key to cycle your target through arena enemy members.
Each time you press the key, you target the next member of the opposing arena team, finally wrapping back around to arena1.
How to Use
Go to the main wow system menu and choose "Key Bindings". Scroll down and find TabArena. There is a single key binding called "Cycle Arena Members". Assign whichever key you wish to this binding.
All credit goes to "Thelibrarian" for his TabParty Addon.
I simply modified it to work for arena1-5.
The original addon can be found @ http://www.wowace.com/addons/tabparty/