If you start to book a flight through the "Fares and Reservations" option, you will get a chance to bring up the "Seat Pinpointer" before you have to actually ticket the reservation. (Choose the class you want to look at the seat map for, of course.) This doesn't seem to work on Northwest-operated flights with a CO flight number, however. Most of the booking engines out there, I think, have a similar feature.
-Adam