I tested in CS6, and things were the same there too. The DPI is really a convenience thing, where you can ask to get an image as if the stage was a different DPI. You'll notice that the width and height figures change. You do end up with a 72 DPI image, but it has more pixels, and if you printed at 300 dpi, a 72 dpi image that had been scaled up by over fours times would still have the same detail a same sized 300 dpi image would have.
In the new image export there is the same idea, only there they went with the less confusing Resize percentage.
Colin - The bug is that the exported image doesn't actually contain more pixels. It is literally a 72dpi image and the low amount of pixels that go with it. The width and height look larger, but the exported image dimensions do not match that. This feature worked exactly as you describe yesterday
I'll give the New Image export feature a shot. Thanks for that pointer.