If a set of very large airtight pontoons were made in the vacuum of space, would they float on top of the upper atmosphere? I thought about this while watching the last season of Mythbusters when they crushed the train tank using a vacuum, and it reminded me a lot of what would happen if you depressurized a submarine under water. My thinking is that if the air pressure is enough to crush the tank, it should be enough to hold the tank up if it was placed on top of the air. My counter though is that air pressure increases too gradually from the upper atmosphere to provide an area that could stabilize the weight in a "floating position".
If they can float and support a significant amount of weight, it might be a good idea for launching spacecraft or low cost (after the cost of manufacturing) transport, as fuel wouldn't be needed to provide lift, just thrust.