Reset/Replace the pivot point of a model.

Put the 3D cursor at the location where you'd like the object center to be. 
Put blender in edit mode(TAB), select all the vertices(A), and then press (Shift+S) then select Cursor > Selection, that will put the cursor at the very middle of your selected vertices.
Go into object mode(TAB), and press the 'object' menu, located at the bottom on default layout, while in the 3D view and select Transform > Center Cursor.

Taken from and edited: http://blenderartists.org/forum/archive/index.php/t-153105.html