I too, wish that the manufacturer would put the payload on the window sticker - that's a great idea! When shopping Fords, I found a dealer in Canada (and then a couple of them in the States) that would put a pic of the red/yellow payload sticker in their pictures for each truck, which made it much easier to get a ballpark figure for the trim/options I wanted.
I can't say how many hours I've spent looking at trucks on lots, opening doors, and saying, "Nope, not enough payload!"
That said, I got my Limited because I realize that I don't haul heavy stuff that often, and I now own an enclosed trailer, so I can tow whatever won't fit in the bed (and still be under on my payload).
Every dealership I've ever walked into when looking for a truck, I've suggested to the salesperson and sales manager that they post pics of the payload sticker on their web site, so I can be more efficient with my time on their lot. Not one has actually followed through with this.
Related stories: A guy I knew from a local cigar shop who owned a kitchen cleaning business and needed to haul heavy payloads. He bought a new 2011 F-150 and his salesperson told him it would haul 3,000lbs. That was so grossly untrue, and he was LIVID when I showed him his payload sticker and it only said 1,800lbs. He went straight to the dealer, fuming, but I guess they didn't do anything for him, because he kept driving the truck. Then, in 2012, a friend of mine bought a new GMC Sierra with the 4.8L V8. He has a 7,000lb GVWR enclosed trailer, and only a 5,500lb towing capacity. Oops.
-John