I've seen a few on domestic flights, but they are really not big enough to be useful unless your baby is under 6 months. The only really decent one I've seen was in the disabled toilet of a brand new 747 coming back from London on Continental a couple of months ago. That one was quite nice. (BTW, they are restricting use of the disabled toilet to people who really are disabled or need it for young children, otherwise everyone would want to use that one, it's spacious!)
The trick I used to use was to put a Huggies Overnight on just before boarding. That will last nicely through a flight, and usually won't leak like regular diapers (reg. ones often leak when a child is strapped in a carseat; too much pressure on one spot, I suppose.) Of course, if you get a major dump, then you have no choice but to change the diaper; I find that the floor in front of the restroom door is a good spot; and then you are in a position to dispose of the diaper properly as well. Do remember to bring your own changing pad.
BTW; you may not know this. FA's are not allowed to throw diapers away for you, because they handle food. If you do have to change a diaper at your seat, fold it up and put it in an airsickness bag, so that it can be properly disposed of as soon as you get a chance.