You play with Eyefinity or Surround gaming, this addon is for you.
MultiScreenSupport move default UI panels to the center screen.

By default all main panels should be center but if one is missing you could still move it using the addon settings.
If you want to know what is the frame name you want to move to the center you could use command line to help you. Simply type when hovering the frame you want to center:

/mss frame

It will display something like that in the chat:

----------------------------
Name: PaperDollFrame
Parent: CharacterFrame
Width: 540.00
Height: 424.00
Strata: MEDIUM
Level: 42
X: 0.00
Y: 0.00
Point: TOPLEFT anchored to CharacterFrame's TOPLEFT
----------------------------

If the parent is not UIParent, it's not the frame that you search to move. Then type

/mss frame "Parent"

Where "Parent" should be replace with the parent value of the previous result here "CharacterFrame"
The new result should be something like:

----------------------------
Name: CharacterFrame
Parent: UIParent
Width: 540.00
Height: 424.00
Strata: MEDIUM
Level: 42
X: 16.00
Y: -116.00
Point: TOPLEFT anchored to MultiScreenSupportParent's TOPLEFT
----------------------------

The name value here "CharacterFrame" should be in the setting panel. If it's not let me know I will add it.
If it is check the option and the panel should move to the center screen.

For all your comments go to the WoWInterface page : http://www.wowinterface.com/downloads/info23821-MultiScreenSupport.html