In OWB and in PPM - there is the Status attribute - if the user sets this to be 'Completed' and there is no ETC, then the % Complete is updated to be 100%. It doesn't work the other way around - ie set to be 100% complete and no ETC, doesn't automatically set the Status to be Completed.
For one PPM client using OWB, I wrote a process to check tasks to see if the % Complete = 100% and ETC = 0, then set the status to be Completed.
Other PPM clients, instruct their PM's to set the Status manually as one could argue that 100% doesn't equate to 'Complete'.