I am having the same problem using AutoCAD Electrical 2008 SP1: Any number entered under "offset" does nothing. In fact, the wire number ends up at the left end of the wire instead of the center even when I pick 'center'. Perhaps you could recreate this bug the way I did it:
I used the spreadsheet-to-PLC-I/O utility to insert some rungs with pushbuttons in them. On the wire before the pushbutton, the wire number is centered, but on the wire after the pushbutton, the wire number seems bound to the left end of the wire. The pushbutton is in the 5th device slot in the spreadsheet if that makes any difference.
Autodesk, I also have some feedback to share about the user interface for wire number placement: I simply want to be able to have the wire number placed at the right hand end of the wire. Being required to specify an offset seems like a less than optimal situation. Can't you guys just give a set of choices like "left, center, or right"? or "top, middle, bottom" (for when vertical rungs are being used)?