You can use this module with the following in your ~/.xmonad/xmonad.hs:

 import XMonad
 import XMonad.Actions.UpdatePointer

Enable it by including it in your logHook definition. Eg:

 logHook = updatePointer Nearest

which will move the pointer to the nearest point of a newly focused window, or

 logHook = updatePointer (Relative 0.5 0.5)

which will move the pointer to the center of a newly focused window.

To use this with an existing logHook, use >> :

 logHook = dynamicLog
           >> updatePointer (Relative 1 1)

which moves the pointer to the bottom-right corner of the focused window.