There's a looooong thread on mobileread forums about how to do this. Amazon changed their ebook format a year or two ago to make it harder to remove the DRM, but someone usually comes up with a new way to do it every time Amazon tries to foil them.

This is the thread I have bookmarked -- I haven't kept up with it all since I quit Kindle back when they removed the "download & transfer" option to let us save our own purchased property. But I think their newest format has been figured out now, so if it's possible to do, the instructions should be in that thread.