The BK Pumpkin burger features two fried slices of kabocha (commonly known as Japanese pumpkin), bacon, lettuce, a beef patty and a creamy, savory nut sauce (with sesame seeds, peanuts, almonds, cashews, and hazelnuts). The sesame bun is meant to be pumpkin-shaped. The price per burger is 310 yen (~$3.90 US).
If two slices of pumpkin aren't enough to satisfy a pumpkin craving, there's the "pumpkin bomb" option which gives you a toal of 10 slices of pumpkin for 100 yen extra (~$1.26 US).
During the promotional period, the pumpkin slices are also available as a optional topping on any burger: 60 yen (~$.76 US) nabs you two slices.
From the sound of it, the BK Pumpkin burger could be really weird or really good or both! I would try it just for curiosity's sake if I ever got the chance. It looks like it'd be hard to eat with the skin on the pumpkin slices though.
Update 10/29/2012: The BK Pumpkin burger is now out and they are now calling the "pumpkin bomb" option, the "heavy pumpkin" and is being promoted as having 500% percent more pumpkin:
