Wow. I actually like the big previous page button... in fact, I think it is annoying that there isn't a previous page button on the right side. I always keep it in the cover and generally hold it with my right hand and have to switch to two hands to go to the previous page. It would be interesting to have snapshots of how people held their Kindles (what with a picture being worth...)
I do agree with everyone that when you buy a book after reading the sample, it should delete the sample and then bookmark you at the same point of the full book.