The title says it all: I have a machine that has several partitions. I'd like to back it all up (preferably as a single image) and restore it to a single partition/maybe LVM. I've run into more and more trouble with the choice of partition sizes and decided it wasn't worth the trouble.
You cannot merge your partitions on a partition level. However, you can make a filesystem backup of your complete system and then restore it in to a full partition. Most backup systems should be able to do this, you can even use rsync or something similar for it. I have also used a combination of cpio and dd commands in the past to do similar things.