You can't use dd on mounted core partitions though, can you? Don't you have to use it from a LiveCD or LiveDVD to be able to back up e.g. the root partition?
I use Partimage from a LiveCD (SystemRescueCd):http://www.sysresccd.org/Main_Page
dd copies partitions without compression, so the backup is as big as the original partition. Partimage allows (optionally) you to backup a partition to a Gzip file, thus saving space.
I'm just now typing a HowTo for SystemRescueCd and Partimage. I'll post it once I've proofread it.