There is a program called pdflrf that does this, and it is far from ideal, since most two-column papers have figures that span both columns. Another tool called PaperCrop does this right, but unfortunately only produces images as output; I have not yet figured out how to reliably join them into e-book format. The non-existence of a good tool to solve this problem is quite annoying...
Just to clarify a point, pdflrf also converts docs to images. It crops & chops, then rasterizes and performs a few image manipulation steps to make the image look better on the Sony screen.
