I have a 'Requester' lookup field (from User Information List) that I am able to pre-select with the logged in user's name using your SPGetCurrentUser() function. That part works great! I can even set the field ControlMode="Dsiplay" so they can't chaneg the selection.Do you think it has to do with what you are doing here? If you make the control display only, you can't change it using any other method... However, you can use Javascript to toggle the control from disabled and enabled. That may make your goal a lot easier over the long run.
Cheers,
Matthew