Nicole Simon informs me, via twitter, that you can VIEW seats while in the selection process.
On the first Select Flights page:
You can View Seats:
Click View Seats:
Look at the little note:
So if you miss this, like I did, when you get farther into the process, you don't see the note or have any way to select a seat. That's still a problem.
They need a step 3.5 - select seats, between step 3 - flight summary and step 4 - payment. A place to select seats. Then, if you don't go through with the payment, remove the seat selection. It's a computer, it should be able to handle it.