Well, I spoke too soon; the 737 is not so bad when it comes to changing tables. We were on a 757 today and found that it had no baby changing tables on board.
This is not of general interest, but for anyone who wonders, A320 lavatories have better baby changing tables than 737 lavatories. If your baby is under five or six months, it probably does not matter, but the 737 table is really short. This somewhat redeems the A320 for me.